I would go ctr. The ubr to me is way to heavy on a carbine type weapon. For me the ctr is light and solid. I just dont see how it gets better. Always thought battery compartments on an ar were a great way to add a lot of weight.
Already got a CTR on my carbine build. The UBR is intended to go on an 18" 6.8 SPC build. I want something with a little more solid then the CTR, but not as heavy as heavy as the PRS (which I have on another AR). From what I've read the UBR is a good cross between the CTR and the PRS in terms of adjustability and being solid enough for precision work.
As it is I got a FDE one on the way, so we'll see.
I put lmt sopmods on all 5 of my builds, I didn't care for the cheekweld on the ctr. That said, I really want to try the ubr on my 18" ar-10 as it seems front heavy and the ubr is quite a bit heavier than the lmt and may help to balance it better.