it would depend really on what you are used to. If you were to try a single 24″ monitor and decide that there is enough space to work with, stick with that. Or, if you feel you need a little more timeline space – the 2 x 19″ widescreens would give more than enough.
Personally, I feel there is enough space on a 24″ monitor; any larger and it can take a while to adjust to the mass of space you have in front of you. I have used a 30″ Apple monitor for editing and it was simply too big to be honest.