The reduced/zero bloom DMR is stronger, the BR is a better balanced utility weapon.
I actually really liked the DMR WITH bloom. I guess it's not too bad though, because the TU won't be applied to the Invasion gamemode, which is pretty much all I'll be playing anyway.
Battle Riffle all the way; nice mid to long range balance; something the DMR couldn't compete in. But for real I did like all DMR swat in REACH, so if that was the setting then I'd pick DMR, but in a traditional Slayer match, BR.