They are aiming at your DS's age whereas Legoland is good for the really little ones too. Therefore even some "scary" rides will take children over a metre. I think it lacks some of the prettiness of Legoland - lots more tarmac and less green. The theming is sporadic - good in places, non existant in others.The zoo and Sealife centre though makes for a nice change from the fair ground feel (bit like the models in Legoland). My DS really liked both and enjoyed watching a talk from a keeper with a merekat and a Kookibara (sorry about spelling). Haven't done the hotel although it looked a bit grim from the outside. Have you checked out Tripadviser? Park would only be just enough to do for two days although my DS wanted to do all the rides 3 times so you can fill it out.Expect it would be VERY busy in holidays so good luck with that.
We went to Chessington a month ago with 3 kids aged 8,10 and 12. ideal for them as the rides were more grown up. there was plenty for us to do over the 2 days. we stayed in the hotel on site which was good. excellent breakfasts but eat out in the evening as the hotel food was not child friendly. also they have lots of little rules which they don't tell you about - no kids in the pool after 8 or before 8 in the morning. They loved legoland 5 years ago but are really too big now for the rides. thought this was a nice bridge between legoland and thorpe park! check out tripadvisor for the hotels. agree with above poster it can get pretty packed high season thats why we went in June