Why not do both? Adding Xian would add 2 or 3 nights to the travel itin. 2 nights really ought to do it - 3 nights if you take your time. Myself, I just did the terracotta warriors and the sights (pagodas, etc) that can easily be found just by pulling up any Xian itin from an internet travel agent. I don't know what other non-tourist activities there are but I'd suggest spending time in Beijing for that, anyways.