I agree with "fedcvgr". I love Daxter but Size Matter is more fun because of the cool weapons, gadgets and armor, and you get to play as both Ratchet *and* Clank (as separate characters), which is fun. Also, Size Matters is more straight forward, whereas in some parts of Daxter, you wonder if you're going in the right direction.
But like "fedcvgr" wrote, Size Matters much shorter than Daxter - I think I finished Size Matters under half the time it took me to finish Daxter. Size Matters is probably too easy, so you tend to zip through the game. But with Size Matters, you can replay the game to get better weapons and gadgets, and there are more extra mini games in Size Matters.
Both Daxter and Size Matters are good games, but i found Size Matters to be a little more fun.
I have always preferred games in the Ratchet series to the Jak series with one exception, this game. I have both and found Ratchet to be boring, and slightly repetitive. I also don't care for the camera controls in Ratchet. Daxter, while maybe a little short is a much better game over all with the exception of the final boss fight which is glitchy at parts, or at least it was for me. Being a major fan of the Ratchet games I recommend Daxter.