My advice is to put the 85mm f1.8 on the 7D--the faster aperture combined with the better high-ISO performance will allow you to capture more "keepers"compared to using the 85mm on the XS. Using the XS with the [slower] 17-50mm f/2.8 coupled with flash allows you to use lower ISOs using flash while still getting decent shots.


That makes the most sense to me, at least.