I prefer the standard A3 24" bull setup. The super bull is ok, but is overkill, in my opinion, and the extra weight doesn't seem to help (me anyway). The 24 does give a little better downrange velocity. The 16 and 20 inch version are also extremely popular, for shooters wanting a little smaller package. Aside from the uppers attached to them, the bull barrels themselves are the same.
My uppers use the NATO chamber, however, we have not found this to be a detriment to accuracy. In fact, the reputation of my rifles and uppers in terms of accuracy shows that. You can ask some of the boys on the varmint hunters board (www.varminthunters.com), too, and they'll back it up.