What do you think? 
Like in terms of replayability and addictivness

Completely different games. However, its easier to recommend Rhythm Heaven.

I think Layton is a far better game. Rhythm Heaven is good, too, but I never had the urge to go back and play it after I beat it- like I do with most rhythm games, so it didn't last me very long. You can beat that game in a couple hours.

I have yet to play Rhythm Heaven, but I will come here once again and say how freaking great Layton is. I am sure RH is good, but Layton is an amazing mix of puzzle, story, art style, music, and many other aspects. It is my favorite game on the DS, basically.

Layton.

Layton seems the more complete game, where as Rhythm is more a minigame collection.
Thanks for the responses

I think their both great games, but Professor Layton can be found for cheaper.

Depends on what you're looking for. 
If you're looking for a challenging puzzler that will make you think and that will wrap you up in an engrossing plot, get Layton
If you're looking for an addictive music game with a lot of replay value, Rhythm Heaven has got you covered.

I don't believe that any game has ever made me feel stupider than Professor Layton. I will say that I was playing it on a friend's DS and they were part-way through the game. The puzzles that I were playing were the ones that they were stuck on. I have it on good authority that a lot of people get severely stuck in several puzzles so I didn't feel too bad.
 
Rhythm Heaven just looks awesome though so you should get that. Word.

I haven't played Rhythm Heaven, but Professor Layton is great. Not for everyone, though. It's aimed at people who like to think. The majority of the puzzles are trick questions.