I'd go with the 5D Mark II because it has a full frame sensor, which gives better image quality and less noise at high ISO. And although high ISO probably won't matter for shooting fashion since I imagine you'd always be working around ISO 100-200... you'll probably find that shooting weddings may require high ISO at times because churches tend to frown on flash photography -- it's unholy or something :) plus it kinda interrupts the ceremony.
As for image sharpness... make sure you're using good lenses. The EF 70-200mm f/2.8L IS II USM is the go-to workhorse lens. You should also consider a prime lens (or two). I believe that portrait photographers tend to prefer something in the 85 - 135 range (e.g. the EF 85mm f/1.2L II USM, the EF 135mm f/2L USM)
Finally, with respect to resolution, The 5D produces larger resolution images. This gives you more freedom to loosely frame your shot and then crop to an appropriate aspect ratio in post-production.
As for image sharpness... make sure you're using good lenses. The EF 70-200mm f/2.8L IS II USM is the go-to workhorse lens. You should also consider a prime lens (or two). I believe that portrait photographers tend to prefer something in the 85 - 135 range (e.g. the EF 85mm f/1.2L II USM, the EF 135mm f/2L USM)
Finally, with respect to resolution, The 5D produces larger resolution images. This gives you more freedom to loosely frame your shot and then crop to an appropriate aspect ratio in post-production.
