Looking at their numbers is similar but the numbers do not tell the whole story, if you actually watch the 2 play you can tell
Gortat (though in a contract year) > Chandler
Gortat makes less than Chandler, making him much more attractive at almost 1/2 his price
Gortat is one of the LIGHTEST BIGS with his footwork/agility with very good length/athleticism
Those skills compliment a PG like Wall, with the PnR and his ability to fake screens and cut b4 the DEF recovers, with the amount of shooters to keep the opponents DEF honest for the room in the paint
Chandler has declined much since coming here however, he hasn't had much help with a horrible coaches philosophy as well
Most importantly John Wall is 10x the player Felton is, which compliments Gortat along with Beal, Ariza, and Nene with Booker, Miller, Webster, Serephin/Gooden on the bench