If you had the graphics card to support it, would you rather have three 20" monitors side-by-side or two 24" monitors as your primary desktop workspace?
I went with 3 x 25" monitors. No complaints.
Depends on screen resolution. If those 20" were full 1920x1080, then the 20's for sure.
I'm currently running 2 x 27" + 17" (would do 3 x 27" if the MBP supported 3 Thunderbolt Displays).
They're 1600 x 900.