From what I read when I was looking at drills was that the higher the voltage you get more power for a given battery size:
14.4v @ 2000mAh = 28.8wH
18.0v @ 2000mAh = 36.0wH
The impression I got (probably wrong) was that the 9.6v was regular consumer grade, 14.4v was more for regular DIY-ers, 18.0v was for DIY-ers wanting more power / low grade professional and 24v was for prosessionals.
I ended up buying a 18v DeWalt as it was only a couple of quid more than the 14.4v version at the time I got it due to it being on offer plus it came with a second battery. Slightly gutted as my father-in-law ended up getting the same one after using mine and he got 3 batteries with his for the same price!