Sea World is a fantastic park! Legoland is a decent park for younger kids, but older kids or those disinterested by Legos may not find it that enjoyable.
If I had to pick one, I'd go with SeaWorld. There is a wider range of shows and rides that should appeal to everyone in your group. Legoland skews much younger.
Another vote for Legoland. We were there with a 9 and 7 yr old and they loved it. Your kids are the perfect ages, you can always do Sea World next time.
Legoland as it just seems like a brighter, happier place! Friends of ours just went there with kids the same ages as yours and had the best day together! Have fun.