The 24LII is softer at f/1.4 than the 24-70mm at f/2.8. But with both at f/2.8, the 24LII is sharper. The 85/1.8 is a little softer wide open than the 24-70mm at 70mm f/2.8, but by f/2 the 85mm is similar across the frame. OOF blur will be stronger with the primes, obviously, since wide open is wider than the zoom. The 85/1.8 is a great value.


Since you'll have time to switch lenses during a portrait session, and you seem not to use the range of the zoom, I'd say go for it!