Depends what you want it for, they are completely different. Jetstream is over twice the weight, has a hood, a little insulation, and pit zips. Mountain jacket is just a wind shell.
I see the use cases like this:
Jetstream - day hunts or backcountry hunts with pack support. Works as an additional warmth layer and cuts the wind, the pit zips help dump heat. Hard to justify the weight on a true backpack hunt but its a versatile layer on active late season hunts.
Mountain - Backpack hunts where weight is important and you just need something to cut the wind and use synthetic or down for true insulation but something that breathes better than a rain shell. Personally gore wind blocker does not breathe well enough for me in active situations without pit zips to justify using the more burly material/cost/weight over a lighter bare bones wind shell (i use a 4 oz patagonia houdini).
The jetstream is my go to winter active layer for casual activities in the colder seasons too. I did a lot of chainsawin in it this winter.