The difference in power between a Mac Pro and an i7 iMac isn't that much. (You did mean Mac Pro and not Mac Book Pro, right?) I don't think they've reved the Mac Pro in a couple years because of this.
Any i5 or i7 Mac will be OK for development work. You'll need to make your tradeoff between portability, power, ram. Apparently having an SSD hard drive is the ultimate in speed. But you probably won't get one if you're looking at refurbs.