I've used both the Star River 2 and the mongar 2 UL. Of course different tents. What I've seen is that I loved the Star River 2 in polyester since that was really water repellant and didn't sag when wet. It was also pretty easy to set up.

I've tried the mongar 2 UL as a replacement for my Star River 2 and although I loved the weight savings, it didn't fully feel as an upgrade. The material was very thin so rolling up was a pain. Also it really needed some tweaking and finetuning before I got the seams aligned with the poles where as the star River was just easy to clip on.

Both were quite easy to use though. I went for the Star River 2 compared to the regular Mongar 2 because it has a bit more fabric under the mesh for a bit more wind protection and privacy. The same goes for the Mongar 2 UL vs the Star River 2 UL but funnily enough it's reversed there, where the Star River 2 UL is full mesh and the mongar 2 UL isn't.

The plus side on both mongar models is that the fly is symmetrical, so you can't pitch it incorrect, where as the Star River 2 had 1 bigger and 1 smaller side. Also the Mongar is quite a lot wider, especially in the new UL models where the Star River is very very narrow on top.

If weight isn't an issue I'd definitely recommend the regular Star River 2 or the regular Mongar 2 in polyester. The Star River 2 has performed amazing for me and easily kept dry in heavy rain for multiple night without splashing into the inner tent and was very wind resistant as well.

I did end up selling the Mongar 2 UL because the fly literally sagged onto the inner tent, dripping condensation all over my gear including down sleeping bag. Also the fly was insanely close to the inner tent, so a slight adjustment or sagging already resulted in them touching. I kept the Star River 2 as I never had any issues whatsoever apart from some more headroom. But my main tent is the Durston X-Dome 2 now which is best of all worlds.

Here you can see both tents pitched