I saw both Percy Jackson and the Lightning Bolt Thief and Clash of the Titans, and I prefer the kids' movie. Although there were flaws with Percy Jackson, I was highly entertained throughout the movie and would happily watch it again. Clash left me totally underwhelmed, and I can't recommend anyone pay full price to see it.
The two movies are the same basic story of Perseus' heroic quest, and the movies both hit the highlights of the Greek myth. Percy Jackson, however, is set in modern times, and I guess this is one of the reasons why I like it more. I love the idea of the Greek Gods being real, but forgotten.
One big difference in the movies is that in Percy Jackson, Percy (Perseus) is said to be the son of Posiedon. Now, this makes for some pretty awesome abilities of Percy's, thereby giving us some pretty awesome fight scenes using water, but the problem is that according to Greek mythology Perseus is Zeus' son. I'm not sure why they made this switch, and I'm not sure I completely care. It just seems an unnecessary choice.