I play combo as the weaker player, and there is no pattern to the preferences as far as I can tell. I play with three different partners and I've played both sides with all of them. Personally, I think returning on the ad side is much more difficult in dubs. I find more guys can hit the kick serve out wide to my BH because that wide corner of the service box is farther from the server, giving them a larger margin of error (at least I find it much easier to hit the kicker out there).
None of these partners expresses a preference--instead asking me which side I like.
Personally, I ask my partner from which side they return best, and go from there.
In mixed I always try to play ad side because that's where so many of the key points originate.