AR15 Barrel length?

I am seriously considering buying an AR15 (or like) rifle. I have done some research and looked at the threads on this site and as a result have pretty much decided on the Bushmaster brand. My question is about the length of the barrel. I've narrowed it down to two models: XM15 E2S 'shorty' 16" or the XM15 E2S A3 20" barrel. The 16" felt better balanced to me but I'm concerned about the accuracy, given the shorter barrel. Would the 16" barrel give me a good grouping at say 200 yards, or 250 yards? I intend to use it for general plinking/varment and some target shooting, but I don't want to short-change (pardon the pun) myself with the shorter barrel. Is there any ballistic data available comparing the various barrel lengths?

I'd appreciate any help on this. Thanks to all who respond.