Why wouldnt my 120 back mount with a slung 40 NOT be better than backmount doubles?
It firstly depends on your requirements and preferences.
But it can be simplified as follows
BM single and pony - okay and easy to rig on standard gear if you have D-rings. As your tank size increases it becomes more unstable like a keel. a pony can exacerbate this. This is generally not a problem of not really noticed much like you don't notice you hands moving and yoru body corrections to maintain balance on a bicycle. However once you go deeper your gas requirements get larger as does your redundancy. Also that pony takes up "real estate" you might want for deco cylinders.
For a travelling diver they either have to schlep their own pony or be able to rent one at destination.
BM Doubles, obv give you more gas (generally) also you're extremely balanced with the weight distribution and their harness is "just" a bigger wing. You have plenty of real estate for deco and they have little impact on boats as they aren't wider than the diver. However you need to be able to deal with the extra weight when exiting the water and ensure the boat ladder can take you. Taking your kit off in the water and expecting the boat crew to haul it up on your behalf because it's too heavy for you really isn't' fair
But you have the additional weight, and you've dedicated two tank to "permanent" use as they're banded and manifolded together. Again not great for the traveling diver unless going to a dedicated tech shop where you can rent them (also harder to transport in your car than singles
SM Needs a dedicate rig which is very different from others. Also it takes time and faff to get fully dialled in. Some boats don't like it because its wider and some SM divers just faff getting in and out of the water. It is nicely balanced, but if you're deco diving then your SM cylinders cut into the real estate for deco cylinders. But for the travelling diver its easy to grab 2 cylinders anywhere and put your slings on them and go. And of course if you're in restricted overheads SM is good
For easy dives I'll use a BM 15l (HP120) and AL 40 sometime I may chose my SM and I certainly take my SM on vacation. I've never dived BM Doubles nor will I ever make dives where I reach the limit of deco cylinders I'm able to carry on SM