FWIW I think Band of Brothers is better.
Band of Brothers is IMO more realistic as far as actual real combat goes than Generation Kill is. I have seen several episodes of Generation Kill and it is mostly young Marines from a re-con platoon advancing across Iraq that is barely able to put up a defense, mostly snipers and booby traps. Band of Brothers is about an Airborne Company of the 101st Airborne Division (Army not Marines) facing a desparate and still dangerous Whermacht (German armored infantry). The 101st Airborne really existed and saw a lot of action during WW2.
Two very different levels of danger as far as combat goes. I really can't say how realistic Generation Kill is, my guess is it is realistic but Band of Brothers is based on the actual true memoirs of a real company that fought in Africa, Italy, France and finally Germany during WW2.
wil.