VirtualBox has full support for VDI, VMDK, and VHD and support for Parallels Version 2 (HDD) (not newer versions).
Answering Your Considerations
- be able to use dynamic sizing
VDI, VMDK, and VHD all support dynamically allocated sizing. VMDK has an additional capability of splitting the storage file into files less than 2 GB each, which is useful if your file system has a small file size limit.
- be able to have snapshots
All four formats support snapshots on VirtualBox.
- be able to move my virtual machine to another OS or even another free virtualization solution with minimal effort (probably something that would run fine on Ubuntu).
VDI is the native format of VirtualBox. I didn't search for any other software that supports this format.
VMDK is developed by and for VMWare, but Sun xVM, QEMU, VirtualBox, SUSE Studio, and .NET DiscUtils also support it. (This format might be the most apt for you because you want virtualization software that would run fine on Ubuntu.)
VHD is the native format of Microsoft Virtual PC. This is a format that is popular with Microsoft products.
I don't know anything about HDD. Judging from looking at this site, Parallels is a Mac OS X product and probably isn't suitable for you, especially considering that VirtualBox only supports an old version of the HDD format.
The format should not affect performance, or at least, performance impacts are negligible.
The factors that influence performance are:
- your physical device limitations (much more noticeable on a hard disk drive than a solid-state drive... Why?)
- expanding a dynamically allocated virtual disk drive (write operations are slower as the virtual disk expands, but once it's large enough, expanding should happen less)
- virtualization technology (hardware vs. software; hardware virtualization helps VirtualBox and improves the speed of virtual operating systems)
- the fact that you are running a virtual operating system. Performance is always slower than running an operating system on the host because of the virtualization process.