Decide what you want to do, then decide what tool you need to do the job. If all you do is hammer in nails, why buy a screwdriver?I currently own a Sigma 50mm 1.4 and use it on my D700.
I keep hearing great things about the 85mm 1.4 and I am tempted to get it.
How useful will the Sigma 85mm 1.4 be for me if I get it?
If you own both with a D700, under what conditions would you use one vs the other?