The atomik is a bigger bike, with a new set of high rise bars, and a softer tall flat seat, and rejet the carb, they make a good all rounder, especially if you want to do alot of trail riding, they will be more comfortable for all day stuff.
if you want to go to BMX Tracks and do lots of "small" shaped jumps then the Pitpro will be more suited, the pitpro is easier to buy upgrade suspension as well since it's more standard pitbike sizes, if jumping or racing is your primary goal this will be a smarter option.
the Atomik is more suited to larger shaped jumps (like on a proper mx track) where the pitpro will cope better in the usual local council BMX Track arrangement. The atomik will be more comfy on 1-2 hours + trail rides and general farm bashing ect.
the reality is the atomik is just budging into the midsize bikes market, or you could call them "large" pitbikes.. if your a tall guy (say over 5ft 10") this may be a more attractive option, just update the bars straight up to proper pitbike riser bars (protaper or similar) and rebuild/cover the seat or buy a predone one and rejet the carb.
the pitpro is still the normal pitbike sized bike, and because of that, most pitbike upgrades will go on fairly easily.
Juls