Harden.
It would be wise for the Thunder to keep Ibaka considering they already have two great perimeter players locked up long term. However, if the Celtics could get one of the two I think we should go for Harden. He's a very efficient scorer with great athleticism, play making ability, and a willingness to defend. Imo, Harden will probably be a wing that nets you 20+ points 5+ assists and 4+ rebounds a game, he already gives you 16/4/4 in 30. Harden could be an all-star, while Ibaka will most likely remain a very high level role player. Bigs are hard to come buy, but if you look at it as something like Joe Johnson (type player) vs Tyson Chandler (type player), I think the choice is clear.
Sidenote: IMHO, the Thunder should pursue Howard this offseason with a package of the two. Otis Smith isn't going to do much better.