From what I've read, despite the theoretical advantages of RAID (twice the throughput, twice the speed, etc.), theories fail yet again and reality kicks in, and the gain is just about nothing! You save two or three seconds off of half a minute of loading, or sure, if you're FRAPSing alot, two dedicated drives in RAID might be better, but it never was intended for desktops, and the gains are so small that it has no place in one. I was going to RAID with my new drives if it was worth it, but it's not.
I'd therefore say a drive with a higher RPM, like a Raptor, would be better. Personally, I don't think its worth roughly a little under three times the cost (
this vs.
this) for not even one and a half times the gain (it's mostly placebo effect versus real results), either, so I'd say none myself, but of the two, the Raptor is the only one that makes sense.
Edit: Going RAID for space is a bad idea, since you can usually (not always, but in most cases) get one larger drive cheaper than two smaller ones that equal it's space.