The benchmarks on the best iMac desktop is not that much better then the 17" MBP laptop according to most sources: c.10400 (MBP) vs. c.11600 (iMac)+.
Get the laptop, then get an Apple 27" screen to go with it. Get the best of both worlds.
Costs more but that's life to get more options, hey ;-)
I have the same setup, and works for me.
(...oh and BTW, if you buy the MBP together with certain other Apple products (eg. screen, time capsule, et al), and purchase the advisable Applecare for the laptop; they all get covered under the laptop policy so you don't have to buy Applecare for each one individually! Which I think is a nice policy on Apple's part — though obviously they get you to buy more stuff at the same time [i.e. also a clever marketing technique] ;-)
Hope helps.
(+ source: MacTracker app [get it it's good!].)