« on: February 06, 2013, 10:08:58 AM »
What a fun dilemma to have! (sorry, but I love this kind of stuff!)
As others mentioned, the 85 has the best bokeh and is an outstanding portrait lens but will be too tight for groups and more general purpose photography. The 50 is overall the most versatile lens, but is a little wide for single portraits, and you'd find yourself wishing for a little more background separation.
I can tell you what I'd do: I'd get the Canon 40/2.8 AND the 85/1.8 USED. I just saw the 40/2.8 show up on the used market for $135, so I'm thinking you could get both for the new price of the sigma 50/1.4. Check classifieds, B&H used, Adorama used, and KEH used.
You'd get the best portrait lens for single/couple portraits and shallow DOF photography, but you'd still have an extremely sharp, compact, stealthy, versatile 40mm lens for everything else. You do lose a couple stops with the 40/2.8, but with group photos (and most wide angle shots) you'd end up stopping down a 1.4 to get everything (or everyone) in focus anyway. You also get better macro with the 40/2.8, but you didn't mention that as a priority.
The only real sacrifice with the 40/2.8 is for very low light handheld, but you've still got your 85/1.8, and now that you've gone Full Frame you can ramp up the ISO more than you might expect. Plus, you've only sunk $135 into the 40, so if you really find yourself wanting the extra two stops on a wide lens down the road, you can easily recoup your costs and grab one of the Sigmas. By that time the 40 will also have taught you whether you'd rather have a 35/1.4 or a 50/1.4.