Well, Gurren Lagann has more fun, in fact, it has an amazing amount of fun and awesome, so much that they throw MUTHA FUCKIN Galaxies!!! (GALAXY SHURIKEN MUTHA FUCKER!!!) Still, you can't really compare them. I mean, Death Note has its good points too. Even if i haven't watched the entire series, I must say it's good. Very good, even. Gurren Lagann's animation is a masterpiece, uncomparable to all, the final battle proves it. Death Note has a very good story and catches you with it's plot, since it's like 100 times more serious than Gurren Lagann. What I must say is, if you're searching fun, and moments that make you cry, go for Gurren Lagann. If you're looking for a serious anime, with a bit of many things, and great themes, then Death Note is what your looking for.
That's all.