It doesn't really matter, especially if you're doing incremental backups. Though, looking on newegg it appears the 7200RPM Barracuda is $10 more...
Anyway, I strongly encourage getting either two backup devices and rotating them from home->work/safe deposit box or backing up offsite using JungleDisk+AmazonS3, Mozy, DropBox, etc.
If you're only worried about physical hard-drive crashes, user error and system errors, then there is no problem with a single backup drive inside your computer. If you're worried about your computer catching on fire, your house catching on fire or your computer being stolen, then having an external or removable backup drive that you can put in a fire safe, and/or two drives you can rotate weekly between home and elsewhere will greatly decrease the likelihood of losing data. If you go the off-site online route (which I encourage in addition to a local backup) I suggest you back up only the irreplaceable things online (documents, code snippets, photos, miniDV videos...), and not the easily replaced stuff (music, movies, programs, etc).
Finally, if you go the external enclosure route (and you're not doing eSATA) the difference between a 5900RPM and a 7200RPM drive is going to be functionally non-existent as you'll probably be limited by the throughput of USB 2.0 or Firewire more than the drive.
Edit: The goodness of backup solutions:
- multiple offsite + [multiple] onsite - Best possible solution
- Offsite + onsite - Great for nearly all uses (computer melts and takes your backup with it, you just get your data from off site)
- Single offsite - Alright, could be better (what if their data center gets taken out by a hurricane?)
- Onsite only - Worst working backup (only protects from hardware/software/user faults not disaster/theft)
- No backup - Don't come crying when you lose your data