It all depends on what exactly you will be doing with the computer. A faster hard drive will make the computer feel "snappier", but the processor will help with serious number crunching.
If you will be doing a LOT of video encoding or other very processor intensive tasks where you will be waiting for the processor, you will be much happier with the processor upgrade.
If you do a lot of things that thrash the disk, but aren't necessarily processor intensive, you will appreciate the faster hard drive more. (For example, Photoshop work on very large files, where disk access is the limiting factor.)
Way back when the MacBook Pro was brand new, I ordered the 7200 RPM drive, but kept the stock 2.0 GHz processor. Now I wish I would have gone with the 5400 RPM drive but the 2.16 GHz processor, as I do a lot of video encoding where the faster hard drive doesn't help at all, but that extra 8% processor speed would. (Not to mention the fact that I'm now woefully short on hard drive space, so I'll be upgrading the hard drive; while I have no option to upgrade the processor, even if I wanted to.)
Of course, there's always adding more RAM.
