Hi Paul,
I tried backmounting with twin 7's and 12's and even inverted the 7s for a while. I'm fully sidemount now and love it. My reasons for changing were not for fashion but for safety and simplicity. An ankle injury made walking with a twinset painful so carrying one cylinder at a time to the water and donning it at the edge or in the water was a logical solution. Underwater I greatly prefer the streamlining and trim advantages of sidemounting but being able to see and control the valves/first stages is imho a MASSIVE advantage by comparison with backmounting. I don't care how proficient a diver is in doing shutdowns, none of us have eyes at the back of our heads, and until the shit hits the fan, we don't really know how we would react in a failure situation, regardless of how well we do the drills. With the valves and regs in direct view, the exact source of any failure is immediately identified. Remember, there is no manifold with sm so having to shut that down as with bm is not necessary - half your gas is safe automatically. The "disadvantage" if you like, is that you have to swap regs over during the dive to balance out the gas use from each cylinder. The advantage of that "disadvantage" is that you know both regs are working perfectly as you are breathing from both regularly. Not the case with backmount.
On the subject of "fashion", yes, sidemounting has become very cool but it would also seem as if the sidemount boom is being driven by the diver rather than by the dive industry (equipment mfg's etc). If anything, there seems to be a shortage of manufacturer support for sidemount and there are really only a few dedicated pure sidemount systems out there - the razor 2, the z harness and the Farrworld harness and a couple of others. I would suggest trying sm before you make up your mind. You do not have to choose sm as a necessity (as cave divers do) but it not being a necessity for the type of diving you do does not mean that it is not an option.
Like all equipment choices, it comes down to what you feel most comfortable using for the type of diving you do. Try it, then decide.
