I might be biased, but I like the URX 3.1. I recently picked up the 13.5" rail for my 14.5" build for $269 which was the same price I bought an DD Omega rail for. I agree with Vlad on the point that the tool is expensive just to install the rail, but since I need to get the muzzle break pinned, I'll pay them another $55.00 to have them mount the URX too. I like the overall feel of the 13.5" URX, its well balanced and has rail where needed for accessories. If you need more rail space I'd look elsewhere as the additional rail panels are only plastic and feel flimsily. The is the one thing I was disappointed about when I got mine.
I am very interested in the BCM KM13. I'd love to try a LW midlength build with that system. I prefer the 1913 rails for now, but I'll end up giving a keymod a try sooner or later.