Slightly off-topic, but even growing up watching both shows, I could tell Dexter was much, much smarter than Jimmy. He has an actual lab that's unfathomably big and actually good inventions. Jimmy's lab is just his parents' basement and all of his inventions were literally random junk stuck together. Plus, pretty much every episode of Jimmy is him screwing things up with his inventions and ruining everything, Dexter's inventions always worked, the zany world and characters around him are what provided the show's plot(y'know, like an actually well-written show).
Dexter hands down, no contest.