Interesting question! I had this exact same issue with my Z4 just 2 weeks ago (I went with 4 new tires).
So I've run both of those tires. I just put A/S 3's on my Z4 and have a set already on my Wife's Z3. I've also run two sets of DSW's on my E90 (the DWS 06's and just the DWS's). So I'm familiar with both tires. The DSW and DSW 06s are great in snow (for an all-season). I've not used the A/S 3's in the snow, but my Wife easily got the Z3 stuck in the front yard in the rain one day (long story - don't ask); so the A/S 3 's probably suck in the snow, just like I've read about.
Since BMWs have decent traction control and stability control, using mixed matched tires is not as bad as it used to be. If you are running mostly in the dry and rain, then based on my familiarity with both tire types, you could probably get away with it. However considering gaps in snow performance between the two tire types, I'd think that might get you in trouble. Also, if you drive real aggressively, you may not want to mix match tires simply because the car may lose some predictability (but it could be fun as long as you don't crash
). Lastly, Tire Rack has a test that includes both the Conti DWS 06 and the Michelin Pilot Sport A/S 3. The Miche's actually have a better dry/wet performance rating than the Conti's. You should review that test and see what you think. Great thing about the Rack is they use current 3 series as their test cars.
If it were me, I'd buy a tire depth gauge, measure the tread depths of the front vs. rear and see how much the rear wear faster than the fronts. If the fronts are at say 5/32nds (halfway worn) and the rears are at the wear bars, then you can be pretty sure you'll wear out the fronts about the same time the rears go again. If that's the case I's go with some more A/S 3's.
You also need to consider the difference in using the same tires but with different tread depths; is that worse than using two different types of tires? Who knows...