Daniel I'm the gear buyer for my better half as well. If she's not comfortable, I'm not going to be...and we're always on the lookout for a great pair of hiking pants. Hope this helps:
Arcteryx uses two types of fabrics in their light hiking pants. One is 96% Nylon and 4% Spandex, like the material used by Patagonia in the Rock Pants. The other is 89% Nylon and 11% PU. The latter material is just a bit heavier, but still very light. Obviously both stretch. Then the pants differ in cut and features.
Arcteryx discontinued the Women's Rampart (probably because it had two weird pleats on each knee, while the men's Rampart is smooth on the knees).
The women's Rampart, Parapet and Palisade Pants are made of the exact same type and weight 96/4 material. They only differ in cut and features.
The Palisade has the most snug athletic cut and an integrated belt and zip cargo pocket. The Parapet has a bit more roomy cut and a zip cargo pocket. The men and women's Rampart don't have zips or velcro on the cargo pockets.
The women's Rabat pants are made of the 89/11 material. The men's Perimeter Pant is made of the same 89/11 material.
We've had good experience with the 96% Nylon 4% Spandex material in the Patagonia Rock Pant for warm weather, so I'd stick with that material from Arcteryx, Patagonia or another brand. The stretch is helpful.
My only beef with the Patagonia Rock pants is that I prefer a bellows cargo pocket to the smooth flat pocket because I actually like to carry things in the cargo pocket. The zip pocket has some room, but you'll feel it if you overstuff it.
Check out the new Rail Riders Ultralight Weather pants, which are lighter than the Weather pants. Don't think they stretch though.