I can't decide between the Gamma AR and the Venta AR. Primary use will be rock climbing. Windproofness is my primary concern, but I also want a stretchy jacket. I know the Venta will be more windproof, but how windproof is the Gamma? Also, which has a slimmer fit? Which jacket is more weatherproof? I wear a medium, but am thin and don't want the jacket to be baggy around my stomach and waist.

If windproofness is your primary concern, then there is no doubt that the Venta is going to win. Even Polartec's marketing material advertises powershield to block about 90% of the wind, while the Windstopper fabric on the Venta will be windproof. Both the Venta and the Gamma AR are advertised as an athletic fit. I own the Gamma AR (I have never tried on or seen the Venta AR) and I can tell you that the Gamma AR in a size medium fits me perfectly - I'm 5'10" and 170ish pounds. If you think you can live with something that is less than 100% windproof, I would strongly encourage you to look at the Gamma MX. This jacket is definitely stretchy, has a good DWR treatment, and it has a significantly slimmer fit than the Gamma AR - it is skin tight on me to the point that I have to size up to a Large to wear it comfortably. The Gamma MX is definitely a more technical jacket and will be the best for activities like climbing. Another jacket that might fit the bill perfectly for you is the Mountain Hardwear Alchemy jacket. Check it out too - windproof, lightly insulated, trim fit and some stretch - it could be a winner for you. Just make sure you read the reviews because it is also a technical jacket that is probably going to be less than ideal for more casual purposes. It sounds like you have a particular use in mind, so you should be able to determine pretty easily which one of these jackets will or will not fit your needs. Good luck! By the way - like I said, I own the Gamma AR and really like it. I'm just not sure that it is the one that will best meet your needs.

I have a jacket made out of the same material (power shield) and it's pretty windproof. Not completely, but it's about as good as the wind stopper material that the Venta is made from. In my experience power shield tends to be a bit colder in wind than wind stopper, but it's not a dramatic difference. Wind stopper is also quite a bit stretchier, from what I've seen. The weather-proofing is mostly a matter of how good the DWR applied to the outside of the jacket is. That's something that's really up to you once the initial DWR wears off. Arc'teryx tends to use a fairly slim cut for most of their clothes and the fit on the jackets is pretty similar from model to model.