Great question, I've been pondering this one too. I snowboard and climb in the PNW backcountry. For the past 3 years, I've owned both the WT primaloft and belay jackets. Given the temps of the PNW, I generally don't put them on when traveling but apply them at rest stops and when sleeping in lightweight bags. So for me, I wanted a lighter option than the Primaloft sweater but still something just as warm. The UL Thermawrap didn't seem to maintain my body heat as well so for 1 oz more, the Western Mountaineering Flight jacket was my choice. My rationale was if conditions were forecasted to be extremely wet, I'd bring the Primaloft sweater but otherwise, I'd go w/ the WM Flight jacket and just keep it under a shell. It's priced ~$100 more than the UL Thermawrap, however so that's a definite drawback but for shaving 40% weight, the WM Flight was my choice.
And re: water resistance comparison between the Belay jacket and Primaloft sweater, I think the Belay jacket is substantially more resistant b/c of the EPIC coating. I've worn the Belay in overnight, wet conditions and didn't notice as much of a depreciation as the Primaloft sweater.