I'm hesitating about buying a new macbook pro laptop.
I just want to learn more about os and get more experience on *unix.
Is the Ubuntu enough to me?
What mac can offer me that Ubuntu can not?
It depends on whether or not you'll use the advantages of owning a mac. Being a Win->Linux->Mac->Linux user, I've established the following:
Buy a mac if:
Buy a PC if:
In sum, and as some others have mentioned, buy a Mac if you NEED a mac(and, of course, if you really like the feel of the laptops/ui/etc). Either way, you should choose what makes you enjoy your computer, whether that be spending less, having tons of choices, or purchasing for proprietary software and fancy/trendy looks.
 sorry for the long post, hope it answered your question sufficiently[/edit]
For me, OS X is the perfect blend between traditional, more user-oriented, graphically savvy world with a great interface (one can dislike it but at least it is consistent and powerful) and the UNIX world of command-line utilities and networking. I've been a FreeBSD developer for quite some years now and I can enjoy both developing and running FreeBSD systems (in a VMware VM) and using my computer for photo-managing tasks with Lightroom and other apps like that without having to switch.
It is also a great system to run on laptops, soemthing which is generally a pain with free OSes (some fare slightly better like ubuntu but I'm not a linux user).
If the goal is to learn UNIX, then something like Linux or BSD makes far more sense than OS X.
When I think of "learning UNIX", I think of something like Arch or Slackware where you start with a minimal base system and build from there. I don't think of a big OS where you install a one-size-fits-all base system with a large closed-source GUI system on top of it. I don't think of a base system where you don't update the packages yourself, but instead get updates rolled up in occasional point release updates of the OS in general (and even then, the updates lag far behind). I don't think of a userland where the accepted practice is to just leave it the hell alone and build your own packages somewhere else, as Fink and MacPorts do.
I type all of this on my brand new 13" MacBook Pro, which I adore. I love using OS X. I don't consider any of the above to be bad necessarily. But learning UNIX? No.
Perhaps the best way to learn UNIX is to throw together a cheap white box to experiment on. Something that you don't need to always be in a working state. Virtual machines are good for playing around with too, and certainly, you can run VMWare Fusion and experiment with other UNIXes in VMs 'till your heart's content.
There's a lot of reasons to like and use OS X, and UNIX is a big one of those, but OS X's UNIX is decidedly prefab. Plenty good for using, but not what I'd point to as an ideal platform for learning the underpinnings.
Buy a Mac, use Mac OS X, get VMware, install Ubuntu.
If you are into playing and editing movies, you'll be surprised how excellent Mac OS handles video. And with VMware you have the option of visiting a Ubuntu desktop to change settings the Linux way while otherwise using Ubuntu from the Mac desktop, using SSH and X11.
I have been a Mac user since 1998 (I was actually a BeOS user back then), so I am obviously biased, but I think you will find that Mac OS X and Ubuntu running at the same time give you a lot more than just running Ubuntu.
Plus you will have access to standard end user applications like Microsoft Office, iTunes, Quicktime Player etc. and Apple's excellent multimedia applications.
Seeing as how you can run Ubuntu on a mac with Parallels for the VMWare Fusion product, I think the real question is do you NEED to have a MAC? Thats quite a bit of money for something to play around with a free OS on....
Mac OS X is a UNIX03-certified UNIX, whereas Ubuntu is a UNIX-like platform. That said, as far as UNIX-level stuff goes, the tools and APIs are pretty similar between the two - they diverge in places like user and network configuration tools, security features, and of course Ubuntu uses X by default whereas Mac OS X provides X inside their WindowServer environment.
If what you want to do is learn "transferable UNIX" then either OS would be appropriate, IMO. I learned generic UNIX stuff on Linux and NeXTSTEP, but I'm now mainly an OS X person after also going through Solaris and OpenBSD. Much of it really does work the same on all of those platforms.
"What mac can offer me that Ubuntu can not?"
All the proprietary stuff, obviously. Off the top of my head, that means Safari, XCode, GarageBand, and a lot of third-party media software. Since you seem to be more of a programmer, you may encounter more software that works on Linux but not (yet) on OS X. Do some research and find out.
As Graham Lee said, OS X has a more Unix-like kernel, but doesn't matter unless you're a purist. Linux systems command more market share if you don't include Macs.
There's also the issue of hardware, as Keltia hinted. If you're a hardware nut who likes tinkering, then Linux is likely the better choice. If you want a computer that works out of the box and is well-polished, then go for a Mac.
And finally, consider the communities. I don't want to get into stereotypes because I know many Mac and Linux users that break the mold in surprising ways, but maybe you care for image. Hard to say.
I have a whole host of posts on the subject: http://regebro.wordpress.com/category/mac/
What is the conclusion? That Ubuntu is a better OS for developers, and OS X is a better OS for everyone else, and that the two are slowly getting closer.
I could have stayed on OS X, if it had better keyboard support. It's clear that the Keyboard still is ignored in the Mac world. Say hello to mouse arm! (Or buy a trackball or something). And there was some other things that annoyed me as well, mostly the was Apple forces you to do everything the Apple Way or not at all.
Macbooks are still OK laptops to use in Ubuntu, although not perfect. Again, lack of buttons in the keyboard is the big issue. And of course, any super-new hardware usually lacks drivers on Linux. It's usually a good idea to buy last seasons model. Cheaper too. ;)
I just want to learn more about os and get more experience on *unix.
Do you want to learn more about all popular operating systems, or only Unix in particular? If you actually also want to learn OS X then you could of course run Linux on a Mac as well (dual boot). However, I feel you should only do that if you expect to use OS X as your main OS. OS X integrates very well with for example the Safe Sleep. After sleep, my MacBook is awake instantly and connects to my WiFi network before I am even ready to use it. The multitouch trackpad also integrates perfectly with OS X. But I would not buy the expensive hardware if all I'd do is boot into Linux.
As a side note: do you actually need a notebook? If you're eager to learn Unix then you may end up setting up some server running 24/7. A notebook is not an ideal server, and actually too expensive as well.
I just migrated (last week) to OS X after using Ubuntu for a while and I feel as if I had put on a straight-jacket. The Mac OS X is a sweet thing, but---from what I've gathered so far--- it is intended to be used with the mouse with very little keyboard support (as another post mentions above). You learn to appreciate the keyboard and the command-line while using Ubuntu, though you certainly do not have to use it for most everything. If you intend to learn *nix, Ubuntu will give you all the freedom you want and the support from the Ubuntu forums site will get you through most of your problems in a breeze. Package management is also simpler than in OS X (fink, macports and all).
Why did I switch? My laptop was stolen =( and I since I was starting from scratch I decided to indulge in sweet hardware =). So far I have been in dependency hell, juggling tarballs, so I am sure glad of the time I spent getting to know my was around a *nix environment with Ubuntu.
Disclaimer: I only talk about Ubuntu and not other distributions because it is the only one I've used. Sure, it is not THE Linux… just another Debian-based distro; as far as I'm concerned, it passed the Mother test (she's happy with it) so it's good enough.