I'll say it from the start : This is one superb OST. There, I said it. Now why is this so superb you ask?. Why should one buy this instead of the "Best Sounds" cds?
Asuming that you like Gurren Lagann as much as the next animefan, you should know that the music in Gurren Lagann is just... Well, superb. And hence, the music on this OST (Still given that you like Gurren Lagann) is likewise. It's filled with catchy Rap songs, quiet serene and upbeat rock. As far as i'm concerned, this OST has everything from the series in the terms of the music... Except from one things : The openings and endings.
The "Best Sounds" cds had the openings and endings in their tracklisting, and that is the one major difference in these two "Soundtracks".
Compairing the "Best Sounds" to the "OST", there are a few differences. "Best sounds" includes the openings and endings plus some "rap versions" of some of the songs... That's about it. The "OST" contains most (if not all) of the songs from "Best sounds", plus more.
As an ending comment, I'd like to point out that the Gurren Lagann OST is indeed a OST. You get the brilliant music from the series, no remixes or anything. Just pure brilliant Gurren Lagann music!.