Tengen Toppa Gurren Laggan (Gurren-hen & Lagann-hen)
I can´t say much about the first movie, as its largely the same as the first half of the show, though i have to mention that the pacing did bother me a bit, as i much preffered the anime´s way of handling it. Also, i liked better how the fight against the generals were done in the show, as it seemed odd for them to be fighting all at the same time, and even with Viral having his own Dai-ganzan. Still, I enjoyed it, and I knew there were going to be problems with the pacing from the get-go, but if you´ve watched the show, I´d say you shouldn´t mind skipping it.
On the other hand, I enjoyed the second movie quite a lot more than the first one, just because less things had to be cut, and a lot was added (or changed). The battle against the Anti-Spiral was made longer, and a lot more grand (even if Super Tengen Toppa Gurren Lagann didn´t do much). One change that I didn´t like though, was how Rossiu´s breakdown was handled, because it suffered, not only of being shortened, and a lot different, but Rossiu, being important as he was, didn´t even get introduced properly in the first movie.
Still, with all of this, i think the second movie is a lot more worth watching than the first one, as it brings a lot more to the series, and even adds a bit more to the epilogue, as little as it is. Still, if you´re going to watch the movies, I recommend watching the anime first, as the movies work better as a summary of what the show is, but it doesn´t show you all that it can offer.