Just had a quick look at the differences in specs. My thoughts, for what they are worth would be that there are three key differences to consider.
The first is the screen size. Bigger screens are like dropper posts on mountain bikes. You don’t think you need them until you try them and then you wonder how you ever coped before. The argument for a larger screen has already been above but one thing you can always do if you do get the 21″ model is add a second screen later. Non Apple screens all work just fine and are much less expensive than the Apple equivalent.
The second thing is the graphics cards, which in the 27″ is significantly better. If you are going to be doing intensive video work, processing large photographs or if you ever want to play any kind of game that has a graphics card requirement, the 27″ model is the one to choose. You’ll be able to do all these things on the 21″, but it could end up being a frustrating experience.
The third thing is the processor; you can opt to have an i7 chip in either model and if you’re going to be using applications that make a lot of use of the graphics capability, then this can also help speed things up. For example if you were doing a lot of photo editing of large RAW files but didn’t do any video editing or gaming, you could compromise by having the 21″ with the i7 processor rather than the 27″ with the graphics card.