Yes, it does matter. When you have the HDD at an empty stage, not much, but when it starts to get full, the performance hit is very noticeable. You'll install FCP in the internal drive, don't forget.
I got the 5400 rpm... I don't think it's worth the extra cash.
However, it's true that when your HDD fills up, it'll get a bit slower. But you should really only be using your MBP for editing. Install all the necessary programs and try to keep everything to a minimal, including storing your music and pictures in another external (or USB key). Obviously, you should NEVER put any video files on your internal... and get the fastest external HD possible (7200+) with an eSata or firewire800 connection.
I just picked up a MBP with 5400 internal HD and I use 7200 external HD's and haven't had any issues.
Also picked up a tiny 5400 extrernal USB2 bus powered drive just for miscellaneous storage and that too works surprisingly well for simple edits when I tested it.