i7, because of the following:
-RAM is easily upgradeable, and DDR3 RAM will come down in price soon. You cannot say the same thing about a processor.
-The computing world is moving more and more into Quad Cores. Soon the Arrandale CPUs will be coming for the MBP (soon is a relative term, next year some time) and then I'm sure we'll just be seeing average computers with Quad Cores. I wouldn't buy a C2D right now unless I had to.
-I've always gone by the philosophy "buy as much power as you can afford". If you're even debating the i7, it means that you have the cash to buy it. I'd go for the more powerful one.
GL