One big monitor or two smaller monitors?

two smaller. you will have more screen space

Two smaller screens. You will end up with more screen space and will be able to multi-task better when you have somethings on one screen and some on the other.

Two smaller monitors you'll have more screen space and technically the two smaller monitors would be bigger.

I would get two smaller ones because that way you don't need to manage your screens real-estate you can just have one task running on all of your screens

Personally, you should get two 24" monitors :). If you can only get two 22" monitors, do that option. Having dual monitors will always be the better option.

I'm always confused about this type of topic, because I did run dual 20inch monitors for awhile and truly loved them, but always never really used both monitors. I recently ordered a 28inch 1920x1200 monitor for my new Mac Mini, since going back to using a single display, my laptop, I find I can use 1 larger display, then 2 smaller displays, since I am very good at organizing the way I arrange my windows. If I like the 28inch monitor, that much, I may order a 2nd one for the Mac Mini, and try it that way, and see if I like it. I typically only run dual monitors, when both monitors are exactly the same, or they don't always line up properly and with OCD, it bugs me and makes my stretched out wallpapers look funny.
To sum it all up, I would say, go with personal preference. I would first try a large display, and then if its still not enough, when you get more money, order another one of those large displays, and then you can have even more real estate over going with two smaller displays.