I recently went through the same process of deciding between Pro2 and Pro3 wheelset… I'm 12 and bit stone, and they were for an "XC" bike (ie. one that gets ridden up hills and down dales, but not raced), but when I say just how flimsy the 24 spoked Pro3 wheels looked in comparison to the 32 spoked Pro2, the decision was easy…
I'd heard from people the Pro3's are a lot flexier too, and the fact that they use straight pull spokes which are a nightmare to get hold of anywhere in the world (Pro2 uses normal DT comp which are available at just about any bike shop), coupled with an inevitably expensive replacement rim (24 spoke DT's and Stans aren't available commercially!), made the Pro3's a no go for me… They're about 60g per wheel lighter than the Pro2's, but the Pro2 wheels are usefully light anyway, and seem a lot stiffer and more convenient as well as cheaper! 60g per wheel is also pretty easy to save by using a lighter tyre, or going tubeless etc. So no real issue anyway, unless you're an XC racer.
Anyway, done a few rides on my Pro2's with Stans 355's on now, and can say they're pretty stiff (certainly stiffer than the 24 spoked versions I eyed up) and still very light! Impressed so far and they're a bargain if you shop around…