Just had a little look at those two, the Marmot looks like a wind/waterproof shell, and the Rab a thickly insulated outer, maybe wrong though.
I previously had a Montane Pertex smock which I didn’t get on with, I got wet from inside and out!
When you say Rab Alpine, which jacket do you mean, as the ‘Alpine’ only refers to the cut..
Edit; just double checked them both, the Rab looks the better of the two, due to the larger (helmet) hood, and more pockets/ vents. Though still £100+.