The primes have faster apertures, but the whole point of a zoom is not that you use the in-betweens (35mm, 50mm etc) but you can use 24mm and 70mm without switching lenses. That being said, 24mm f/1.4 is a nice thing to have, as is 85mm f/1.2 (as there is no 70mm prime). But that'll cost you more and you will have to switch lenses. If you are walking around with a lens than having a 24mm prime and 85mm prime is a bit stupid. But for shots that you know you'll be taking and have the time to switch lenses that the primes would be better. The best option, obviously, is to have have the zoom and the primes.


EDIT you said you use the 24-70 for portrait sessions; in that case the primes seem like the better (albeit more $$$) idea.


my 2¢


brendan