At your ranges, there is no reason (other than personal preference or the dreaded, unavoidable "maximum fragmentation range" arguement) to go with the longer 20" barrel.
Although a couple of friends and I have successfully engaged (read "hit"
) a steel sillouette at 500 yards with both 14.5" and 16" rifles, I'd start leaning towards the 20" after 350 yards or so.
I also can't let this post go by without a plug for my FAVORITE rifle of all... the 16" lightweight Mid-length! I know that because of your bullet weight stipulations, you'd have to wait for that 1:7 "group buy" or some other source, but I just REALLY like the Middie for looks, extra forend length, less gas impulse, and did I mention looks? I think they're really worth the extra effort.
*edited to add plug for mid-length