If you want to be part of the public beta of retina display OSX go for the Retina MBP. Otherwise you get more performance for your dollar withteh iMac. Even if you can't really upgrade much on it beyond the ram.
For me... I'd go iMac but that's a personal preference.
I did the whole "MacBook Pro as a Desktop Replacement" for a a while, and it wasn't as nice.
More cables lying around
Still used external monitor since I need more than 15" for desk work
More desk space taken up with keyboard + laptop + monitor than even a regular PC
Now, if you plan on using it as a laptop from time to time AND either (you aren't flush with OR you only want 1 purchase)
Then... maybe the laptop is the smarter bet.
In my case, my purchase will JUST be a desktop so I'd rather have 1 huge screen, with very few cables, taking up less desk space. My laptop is still "good enough" to serve as a laptop... and if I were to replace it I'd just get a 13" bare bones MacBook Pro
The 27" iMac is high enough resolution for you to do some real work, multitasking, and still be able to have decent gaming experience. If you still crave retina then get yourself an iPad 3 as a companion to the iMac, it'll compliment it nicely. Get the iMac from apple refurbished. When all is said and done you'll still have some nice change in your pocket.