Probably depends on the space you have, I think.
I have a small desk in a rather small office area in my apartment. I got the 20 inch because of my space considerations, and let me say - I'm glad I did! The 24 inch would have been way too big and would have made me feel boxed in and cramped if I had gotten it instead.
If space isn't an issue, consider what you'll be using it for. Lots of graphics? Pics, videos, movies, etc? If space is not something you have to take into account, and you expect to be doing a lot of graphics heavy work, a 24 inch might be better.