I have two dell monitors, ones a 24 and ones a 22... it's good to see how things look on different monitors, but to compare things side by side it's impossible... if you want to have the same image on both monitors to compare subtle changes in the picture that you've made you'll want two of the same exact monitors. For modeling I only really use one, have the other one open with the web browser or a tutorial, or keep my music over in the other monitor so I don't have to keep minimizing and maximizing.
I always say larger monitors are better for anything with graphics. If I could upgrade I'd get two newer 24" I'd like to get 32" but I'm too close to my monitors and it'd hurt my eyes.