The 27" has the option for Quad Core processors while the 21.5" doesn't, but has a pretty fast Core2Duo processor. So you'll need to look at that, and also that the graphics card is a little lower on the scale on the 21.5"..
If you're tight for money, the 21.5" should do, but if you can go to the 27", that'll be quite a boost..
Regards