I don't tend to swap uppers around too much. I'd just build a complete, dedicated rifle. But there are factory rifles cheaper than I can build something right now.
I'm kind of liking the Rock River A4 Varmint model in either the 18" or 20" model.
DPMS offers a good looking rifle in their TRP with a stainless 20"'barrel but I can't find any real good info on it's accuracy potential and not sue I could bring myself to spend $1K on anything DPMS builds.