My advice would be don't compare the two. Go to Thorpe without a set idea of what to expect. But from my own personal view point, I think TP is vastly better than AT. The fact you don't have to walk miles just to get from one side of the park to the other is definitely the one thing you will really notice. The food options are basically the same, but there is also branded food outlets at TP. These are Burger King and KFC (That might have changed, This is going by last years information as the park hasn't opened yet). Yes there is no single rider queue, and the batching can occasionally be horrendous, but its all good fun. The park is easy to move around. Its basically a funny shaped circle and I think it actually more atmospheric than Towers as its not so big.
There was a time as recent as this time last year where I would argue that Alton is better than Thorpe, but following cut after cut at Alton, it is obvious that, in terms of things such as operations, value, availability, variety, and quality, Thorpe is by far superior. It is getting to the point where I am starting to think of Thorpe as Merlin’s flagship park, to the point where I have two 3 day trips booked in the space of a month, compared to one 2 day trip to Alton.
Going back to the main point, as Dom said, don’t go with a preconceived idea that Thorpe is a southern Alton Towers. Yeah, they have similarities but overall, they are quite different.