So adding this for posterity in case anyone else goes down this path. The RAB positron and the Montbell alpine showed up. Here are some quick impressions:
Montbell alpine - black - XXL $300 from Montbell - Fit is good. Not quite as roomy as I want but close. Easily handles a soft-shell underneath and more room to layer. Design is fine, if not a bit "classic". Two handwarmer pockets is all. Sleeves are plenty long. The XXL is ~21oz. It is noticeably less "puffy" than the RAB, ie the loft is less thick all around. It is also a lighter weight and by appearances more delicate outer fabric. I feel like it is a good jacket but not enough of a jump up in warmth from my lighter down jackets to make me completely happy.
RAB positron - Aspen (green) - XXL $200 from Moosejaw / Mountain Steals - Fit is not good. A bit slimmer throughout the shoulders, chest and waist. I am on the husky side, but don't have much of a gut. If you do, this is not your jacket. The key issue for me is the arms. The length is good but the upper arm is too tight. If feels snug with a tshirt and tight with layers under it. it constricts my elbows when I bend them. I lift but I am not toting around huge guns, but this doesn't work, so unfortunately, it is a deal breaker. Apart from that, the outer fabric is heavier and appears more durable, like a ripstop type nylon. It also has treated down and more of it. The downside is the XXL weighs ~31oz. Has two handwarmer and one each chest pocket inside and outside. Design is slicker and more modern (newer hardware, hood adjustment, etc.) in a way that enhances it a bit but probably doesn't truly impact performance. The Aspen color is just a shade or two from obnoxious. Kind of a lime green or the inside of a ripe avocado. The interior lining is mustard yellow.
Verdict: the RAB is going back due to fit. I like the Montbell but don't love it so I am going to keep it for a few weeks while I wait on an OR Coldfront Down Jacket and Marmot Mens Guide Down Hoody. Were it not for the fit and slightly obnoxious color, I think I could have been happy with the RAB.