15" for sure~! You stare at that screen every single moment you are at the computer, but you only need a slight speed boost every once and a while. Isn't a few seconds here and there worth the price of getting a much larger screen. Have you sat for a while infront of a 12" screen. The only reason I see to get a 12" is for portability...period. Since that is not an issue here, get the much larger screen.
If you only have one window open all the time and don't multitask, etc, maybe the 12 inch will work...but don't buy without working for a while in front of one (or mask your current computer to show 12" with paper) cause you will regret it if you have not tried one out. It is way small.
Johnny