There is no doubt that 7500 will give you better performance. It will def be worth the additional investment.
Good Luck, Stan
For video, you will want 7200 RPM.
If you are planning on doing anything, except archiving, you will want at least FW-800 (IEEE-1394b), or better yet, eSATA. Many have had horrible issues with USB 2.0 externals, when trying to edit to/from, as the connection is too slow for the throughput that video editing requires. These problems are in read/write errors, including the dreaded Delayed Write Fault, which can easily corrupt your Project, your Assets and might even corrupt the entire external. FW-400 (IEEE-1394a, though often the "a" is omitted, and the name might also be iLink) seems much more stable, BUT that connection has proved too slow for my use. I use all FW-800's, and will also add eSATA on my new machines.
You mention using the drive for backup and for editing... those are kind of two completely different requirements, really.
If you're simply doing backups at night, where you don't care about speed, you can pretty much use anything - a plain old USB connection with a 5400 RPM drive would be fine as long as it's big enough to hold everything you want.
That's pretty much what I have: a 1.5TB fireproof, waterproof, shockproof, USB-based drive. It slow, but who cares? It's big and bulletproof - perfect for backups.
However, if you want to do editing using the external drive, you'll want at least 7200 RPM, but more importantly you'll want to make sure it's using at least a Firewire-800 or eSATA connection. Using USB is pretty much out of the question, and Firewire-400 would be disappointing. IMHO.
Beat me again.
At least we're on the same page!