Depends what you like really, ive been to both and theyve got some awesome rides
AT - oblivion, rita queen of speed, air, nemisis, ripsaw, some thing where you go about in a car thing and shoot zombies with lazer tag guns, ohh and that new ride 13
Thorpe park - this fvck off log fume thats like 75-100ft high think its called tidal wave, and boy does it make a hell of a wave when you get to the bottom! Collossus (world first 10 loop rollercoaster) river rapids, no way out (backwards in the dark roller coaster) and the new saw ride which looks nuts!
Thats allni can think off with the rides but in terms of awesome ness id say thorpe park cause most of alton towers roller coasters are the same, airs like nemisis except the leg dangling cart tilts like your flying, rita just goes fast theres no loops etc
Where as thorpe parks rides have more excitement, collossus has 10 fvcking loops! The saw one makes you think your going to get chopped up based on the advert, no way out as i said goes backwards and its in the dark!
Hope this helps a bit

lol