To answer your question- There are several reasons.
First, the U2410 would become my primary monitor. The primary monitor should be as big as possible.
Second, I find that when you are trying to select an item 9for extraction), the larger the screen the easier it is to choose the right edge. Third, I have a 20" as my second now. I hate how the two monitors don't line up. I find it dizzying going back and forth between the two of them when they are not lined up. My eyes jump up and down. That is why I want another Dell 24". They will line up perfectly.
Also, I often have three or four windows open. On the second monitor (24")I can have two active windows open and ample room to view them.
Lastly, on a purely cosmetic point, two 24" monitors look better mounted on the arms in my office than two different sizes.
Canon 40D and more lenses than my wife knows about...