If you go for the 15" seriously look at the retina option. the image quality of the screen is just gorgeous .
However if you opt for an iMac you could go for the 21 " and save a few hundred quid over the 27". However once you see the 27" game over.....
Also hang on for a few weeks if you are going for the iMac as new models are slatted for December. The new ones are ultra thin ( at the edge ) and have USB 3 and Thunderbolt ports, rather than USB 2 and Firewire.