SB, the 3805 is more advanced in its features and should be the better pick. If you're mentioning the M3 or M22 in connection with back surrounds, as I replied before, the M2 would be fine, but the M22 would be a bit better match than the M3.
There's no problem with any receiver in handling speakers of different impedance simultaneously. The amplifier sections are separate for each speaker and the ones that would be driving 6 or 8 ohm speakers simply have a bit easier job than the ones driving 4 ohm speakers.
