I have two Colt uppers:  One is a 16" A2 upper.  The second is a 20" A3 upper.   My question is which one to keep or interchange with the other.  At this point optics (ACOG...) is down the road, but I would prefer the A3 upper.  I am somewhat undecided regarding the barrel selection.  Looking for comments with respect to bbl selection...

Thanks.

The only advantage I could see for the 20" upper would the increased radius of the iron sights leading to a bit better accuracy. Otherwise, I don't think that extra 4" does much for the performance of the 7.62x39. The 7.62x39 isn't a long range performer anyhow so high power optics on that 20" barrel would just be a waste in my opinion.

Here's what I would do. Swap the 16" barrel onto the flat top upper. That way you can have a lighter, handier, more compact 7.62x39 AR15 carbine with flexibility for different optical sights. In fact, I have this exact set up and really like it. Install the 20" barrel on the A2 receiver and enjoy it as an iron sighted full size rifle.