MScorpa, I've done single-tank sidemount (STSM) before and I didn't mind it. As long as you're diving an alu tank, the weight shift isn't bad. You end up hanging tank-side low for a bit at the beginning and then tank-side high at the end. I always use a frog kick, and there's a way of frog-kicking with your feet slightly out of alignment to allow for a twisting moment to be applied to compensate. I use that to compensate for my imbalance, and slowly migrate to the other side when the tank starts emptying. The guy I first started doing that with would carry a weight belt with unbalanced weight on it, and spin it during a few points through the dive. I put a 2# weight on the bottom of my AL80 for tank trim, and I do the same on my non-tank hip just to compensate for that weight....but I don't use lead to compensate for the tank's buoyancy.
Glenn, I've tried mismatched tanks (LDS was doing a pool dive, and I paid for two identical SM tanks. One was at 3000psi, the other was at 14.7 (completely empty). I had a steel 72 on one hip and a LP108 (I think) on the other. I was NOT a fan. I ended up unclipping both and putting the one with my longhose at the bottom of the pool so I could continue to help my buddy out (we were tweaking gear). I will tell you that anything other than a "standard" alu tank (neutral at half-full) is an absolute PAIN to dive offset with.
Having said that, I've pretty much given up on STSM. My fiancee, a buddy, and I did a two-tank boat dive a couple of weekends ago. We all had two AL80's for two dives. They used them as BM singles and carried one on each dive. I used mine as traditional two-tank SM and simply had the redundancy afforded by having both tanks. At the end of the day, the dive op didn't know any different. Well, I told them....but, like, it was the same as far as they were concerned. I dove off of a boat, and didn't need any DM help either way....just a good buddy. I see no need for STSM, except for in rare scenarios. eg: Most diving in Roatan is one tank off of a boat, and you switch tanks between dives as you come back to shore. In that scenario, I'd be very tempted to just SM the one tank.