Get a Firewire model, preferably a Firewire 800.
This of course depends on what type of Mac you have and whether it has any Firewire 800 ports. Not all Mac's do.
All external drives that are connected via Firewire can boot the machine as long as they have a valid operating system installed on them.
The Intel Mac's can also be booted from a USB drive with a valid OS installed on it.
I like La Cie as a complete package for drives but that is a personal thing. La Cie only make the cases, they do not make the hard drives. Seagate/Western Digital/Maxtor are all good brands of drive. The Seagate has a 5 year warranty which, I know from experience, they honor.
It is possible to put together an external Firewire drive by purchasing an enclosure and drive separately. You can often make major savings in that way. You may have to search to find a Firewire 800 case but they are not that difficult to find.
I'm sure there will be other comments forthcoming.
I want to buy a external drive for use with OS X.
About the only thing I am sure of is the size...500 GB.
Should I buy a FireWire or USB model? (cost is not a concern)
Do I need one that is bootable? How can I discover if it is bootable?
Any recommendation as to brand of drive would be appreciated.