Ditto for Unrepentant_Sinner. I have both rifles you are talking about, the 16" M4 and a pre-ban 20" A2. The M4 is definitely a cool rifle, but I shoot better with the A2 for reasons mentioned by Unrepentant_Sinner. I like the longer fixed stock for a variety of reasons. It is a bit longer, doesn't wobble and the butt plate has a slight angle and is bigger than the standard collapsible stock. If you do get a tele-stock, get the new version with the bigger butt plate area which is also angled to provide more stable shooting.
Also, the A2s sight radius does help shooting for me, as the M4 front sight post is very close to the rear sight and not as precise.
In the army I shoot with the M4, and it is necessary to shorten the stock when shooting with PASGT body armor, in fact I had to shorten it to the second shortest position.
If you can't decide, do what I did, get both. You may be broke, but it is lots of fun.