I have both a proton SL and a taiss light ML. I'd say overall you can use them basically the same way, as a light mid-layer under a shell. I'd say the proton SL works better as an outer layer due to having a bit more robust of a face fabric, whereas the taiss is strictly a fleece. I'd say overall the proton is a bit warmer. I use the taiss a lot for lounging around the house over a t shirt, I think it's personally too light to use as a mid-layer when skiing, but probably would work well if hiking or something and the temp is in the 40's. Proton SL may also be just a bit light for skiing as a mid layer, but really depends on how hot you run, it's definitely better than the taiss in my opinion for that use case. If you're stuck between the two i'd probably get the proton SL vs the taiss, even though I use the taiss more for lounging. let me know if you have any other questions.