Have to buy external HD for a friend's iMac. Any reason to buy one bigger than her internal drive?

[GeneL]

I still have to check on the size of her internal storage. The question is would a bigger external drive that she could boot from be a lot slower than her internal drive and, if so, wouldn't it make more sense just to match hers in size and have her keep booting from the internal drive?

I bought a 300GB miniStack for Mavis' intel mini and have her booted from it and back up to the internal HD. Seems to be an excellent setup for her.

The next question is, should I get a miniStack, with its many neat features *(i.e. fast quiet operation, extra ports and small size) for my friend, whatever size is appropriate, or should I look at other external drives, which may be cheaper for their size and only USB?

Also, OWC advertises their 500GB minStack as being bootable, both FW and USB. I thought only FW externals were bootable. Am I wrong or do other USB external drives boot OSX nowadays? If so, I think there might be other cheaper solutions.

Suggestion and/or recommendations?

Thanks for your thoughts and expertise,

GeneL