I liken GunBuster and Diebuster to Chrono Trigger and Chrono Cross.
The originals were loved, and the ones that came after were good in their own right, but for different reasons that the first. They don't have a whole lot to do with each other, but the threads that tie them together, and the Easter Eggs, etc, are cool if you liked the original. They both have "HOLY SH-" moments that tie back to the originals, but aren't meant to out do what came before.
I like Diebuster, I think the original is better, but I'm ok with it for what it is. The ending was awesome, as was the reveal in ep 4 with the Space Monsters.