I'd go for the bigger machine. A 27" screen gives you lots of real estate for modelling. Not sure about AMD cards, but then you don't have any choice. The I-7 is a faster processor but has been superceded. The new Intel ones are not available for the Mac yet.

If you are buying an I-Mac, you are not trying to save money. If you were short of money, you wouldn't be buying a Mac, as performance per £/$ etc is not as good as a PC. There is nothing wrong with Macs and a lot of people prefer OSX for usability etc. I have a MacBook and like it but my PC laptop outperforms it and was cheaper.

Edit: Don't buy your extra RAM from Apple. It is way overpriced. Buy it later from Kingston, etc.