Band of Brothers was certainly more polished--no doubt. It needed to be, given the subject matter.
The best thing about Generation Kill was that it had a very unpolished, gritty feel to it, just like the book. The book communicated how chaotic and fast-paced the OIF land operation was; how confusing many of the situations they faced were; how there were many occasions when the guys were not comfortable at all with what they were doing. I thought the manner in which they made the series matched that. There was no real theme music, just the crypto chirp and transmissions of a SINGCARS radio. The credits almost looked like an afterthought, you having just been thrust out of whatever confused situation the platoon was in during the episode.
What I'll say is that if I was to make a "war movie" based on my own experiences in the Marine Corps, it would look and feel and sound a lot more like Generation Kill than Band of Brothers--on purpose.