USB or Firewire no diff as long as the mac are able to support
it. The only thing to take note is when you format it you cannot
format it as NTFS format cos i don't think they can support (maybe
i'm wrong).
And advise on buying an external harddisk is you can buy a external
harddisk that is by itself or you can make one using either a
laptop harddisk or a desktop IDE harddisk with the appropriate
casing. If you are going to make one, try don't go for IDE because
it generate alot of heat and without a good casing, it may burn
your harddisk sooner or later (proven).