Gasol is much better than Val though. Vananciunas is a volume scorer, but that's about it. He doesn't see the floor well, doesn't pass well, doesn't block out well, doesn't play or defend well outside the paint, doesn't run well, doesn't shoot well from long distance. Gasol is a very high IQ guy, does all those things, and just simply fits better on the team matchups-wise with Ibaka than Val did.
What I love most about Gasol is his ability to find the open man. He makes guys around him better, gives them open looks. Val sucked at that.
Also advanced stats love Gasol much more than Val for many of those reasons. Take VORP for example.
The only thing that sucks about the trade is losing Delon. He's a good player.