Well... Hard question... They belong to two extremaly diffrent worlds so judging who is better between those two is pontless... I can say Spongebob is funnier for sure. And more stupid, naive... But Naruto also had his funny moments and moments (sorry for repeating word -I can't think of a equivalent) of extreme stupidity and naivness.
If the question was "Who is more annoying/stupid/inmature/anything like this" I would vote for Naruto - he's just an insolent brat :3.
Well, I believe Spongebob is a total Retard when compared to Naruto (That's saying something!), So It's Naruto FTW!
BTW, Why is Spongebob drawn with holes all over his body? (Don't answer this if you're a Straight man.).