What is most important is how you are going to use the external drive.
If you use it to store media files such as movies then you just read and write very large files. Reading these is usually a lot faster then you can display a movie. This means low noise and possibly low power are important. Even an old 5400RPM drive will suffice qua speed. And a low RPM drive is likely to make less noise than most other drives.
If you use it to store a large number of small files, randomly accessed then it is a totally different story. Access time gets very important. That depends on how fast your disk platter spin. (A drive only reads data below it read/write head. If you want to read something just head of the head than you are lucky and you will have your data in almost no time. If the data has just passed the head then you will need to wait until the disk has spun almost a full circle. With multiple accesses this balances out and you will have to wait an average of half a disk rotation for new data. Faster rotations help.
I wonder what is more important for portable external HDD, interface (USB 2.0, USB 3.0, firewire, etc)
The interface also depends, but.
- USB1. Ancient. SLOW. Do not use. (Copying a 40GB disk takes a full weekend)
- USB2. Universally used. Not very fast. Has significant overhead in the protocol. Not easy to access unless your OS has drivers for it (which any modern OS has. But still anoying when booting rescue disks). It only real advantage is that every PC has an USB interface.
- USB3. New. Found only on a few new computers. A lot faster. Interface speed is good enough, though it has the same other disadvantages of USB2 (protocol overhead, drivers).
- Firewire: Found on only half of modern PCs and laptops. Usually somewhat faster than USB2. Has some nice advantages over USB when streaming multimedia files, but for HDD access these are not important. A firewire interface is found on a lot on professional euipment, but due to royalties only half of modern PCs seem to have this interface.
- eSATA. Often as fast as the drive supports. No additional overhead. Works always. Technically the best interface for a HDD since it's the native interface of the drive. Like USB3 it is not present on all computers. eSATA does not need any additional drivers. Ever. You can just remove the drive from a existing computer, plug it in externally and boot it without problems and without modifications. You can boot ancient DOS and access the drive. I think every professional IT person loves it. As do people who need pure performance.
- Thunderbolt: Not really a drive interface. Think of it as PCIe via a cable. You can plug a SATA card in your PC and plug a drive into that, or but all of that behind a cable and archive the same. It will not work better then eSATA, but the interface can also be used for other stuff (e.g. external GPU, external RAID (incl. HW RAID card), serial interfaces, USB interfaces, you name it.). This means that while for pure HDD access it is second to eSATA, it is a lot more functional. This makes it a good choice if you do not want a dozen different connectors but still want very high speed.
ll of these interfaces have a published speed. You will never reach that speed for data transfer. The published speeds are usually the 'wire speed', or the speed on which signals can be sent across. The makes sense of these signals you will need to decode them. Usually this causes about 25% overhead. eSATA is the only option with the least unnecessary overhead. It is also the least flexible (e.g. you can use to to communicate with a HDD or a CD/DVD/BD, but you can not talk to a camera or keyboard or … )
or rotational speed (5400 rpm, 7200 rpm, etc).
For pure sequential access (read: large files on a defragmented filesystem) almost any drive speed will do. The faster the rotation speed the faster the drive can read data, but unless you use a very fast interface that speed will be capped at the speed of the (slower) interface. In which case the difference is not significant.
For mostly random access: Get the fastest rotation you can afford, or get a SSD.
If you use eSATA or thunderbolt: Get a SSD. In these cases your interface will not slow you down.