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] ;-)
(+ source: MacTracker app [get it it's good!].)
13" v 21.5" => not much "immersion" in either size these days, so not much difference.
There is a large difference in the benchmarks between the top and bottom processors available in any one range. eg. 13" 2.3GHz is just 5900 vs. the best 15/17"s do X1.9 that. So spending that bit more really does make a difference in, say, 3 year ownership term performance levels. ie. Remember you will likely own the machine for a few years so judgement is for next year and the year after not just the next 12 months.
(Again, the free MacTracker app, on iOS and OS X, has benchmarks and prices for easier comparisons — build quality very good on either, so the rest is down to personal taste I'm afraid.)
...oh, what's that? That's OK, your welcome, any time!