Is the RPM of the hard drive that important?

I would opt for the 7200RPM drive, just find one with as much storage capacity as possible. I recently upgraded to a 7200RPM from 5400, and I noticed a difference in speed as far as opening apps and such. Not a big difference, but noticeable.

You may be able to quantify your options a bit, assuming you have a mac already.

Set your dock to not auto-hide and put Activity Monitor (from Applications -> Utilities)in it. Right click on the icon in the dock and choose Dock Icon / Show Disk Activity. Now open the programs you think you'd use and do what you expect you would do in them, keeping an eye on that icon. Now as I understand it OS X keeps closed programs in memory for reuse if you open them up again until it fills up the existing ram, then it starts recycling (witness the dot under your Safari / iTunes after you close them) so you could expect that after the initial program load your 8 Gigs of ram will work to minimize the work your drive has to do.

On a somewhat related note, it's not uncommon to kit out a performance / gaming rig with a small HDD or even SSD for the OS then add in a large capacity (slow) internal or external drive for your music, movies and other programs where speed doesn't matter much but having them available does.

There would be a fairly noticeable difference, however you have to decide if the cost is worth it to you, it's noticeable, but not that significant.