I have an AN/DP course scheduled and Cavern scheduled and both instructors require backmount. My desire is to learn the fundamentals in backmount. From my VERY LIMITED sidemount experience it seems skills are harder in backmount. Exacting buoyancy control is harder in backmount. Propulsion is more demanding in backmount. I think if I learn the hard way, the easy way will take care of itself. Plus I paid good money for my double 104's and I want to get some use out of them. A year from now they may be split into cylinders for sidemount.
My trim and bouyancy were flat out ON, when I moved from a single tank to backmounted doubles. This was not the case when I moved to sidemount, it took much more tuning for me to get "true".
Depending on your sidemount rig, propulsion may not be any better. Lots of people consider sidemount less streamlined. Me? I personally didn't see any difference at all in streamlining or propulsion.
Valve drills may be the only thing I can think of that would be any harder in backmount. Sidemount will actually add a slight bit of task loading(which will become natural with practice).
When you split your 104s, those bands and isolator bar will still sell for perfectly good money!
I'm with Dive_Aholic. If you know that you will be diving sidemount in caves, why dive in a setup now that will teach you muscle memory that you will have to unlearn later on? Its easier to learn a muscle memory than to relearn a new one after a set of skills have already been established.
I guess it really depends on how much you think you'll dive BM doubles... If you plan to continue on with doubles in the future for other aspects of diving, probably not a bad idea, especially if you are already diving that way. If you plan to fully drop doubles for sidemount diving, I think you are truly wasting effort.
Sidemount diving is easier to get into the water and it makes tighter caves accessible. I wouldn't say its any easier in any other aspect, so if you are looking for the tougher way to cave dive first... I think you got the wrong idea.