In reply to iamaclimber:
I'd discount the Montane Flux due to the inferior insulation. (approx 75% as warm as gold)
Your best options for a 100g belay jacket are the ME Fitzroy or the Rab Generator Alpine.
The Arc' Alpha SV isn't really a belay jacket - more of a very warm midlayer.
If you run very cold or are spending lots of time belaying, your best options are the ME Citadel, Patagonia DAS or the Montane Spitfire One - difficult to choose between, however I'd probably go for the Montane (only because it's slightly warmer (240g primaloft one in the chest compared to 200g in the ME) and the hood cinches come out on the inside of the jacket and therefore won't whip you in the face when you're on a belay and the wind is howling).
Regarding the Rab Gen Alpine vs the ME Fitzroy the main differences are:
- Rab packs into own pocket (better) / ME has stuff sack (can fly off in wild conditions)
- Rab has a popper at the bottom (very useful when belaying) / ME doesn't
- ME has fleeced lined pockets / Rab doesn't
- Rab hood cinches come out on inside (so you don't get whipped in the face) / ME comes out on outside
They're both excellent belay jackets though (the Rab may have the slight edge - only just).
Source: I own a ME Fitzroy
Post edited at 16:28