Well I'm taking a swipe at devon because he made a crack about what I was asking without offering any useful information.
So clearly SM is better for at least some profiles of caves. SM is generally easier on the back. There's a large overlap of dives where it just doesn't matter. They are both tools, but I don't hear a lot about where the real limitations are. And I've listened to Boegarts get asked this question and he didn't really have an answer, either, and in the past 2 years since then I haven't seen it answered anywhere. So what is the reasonable amount of tanks to be carry sidemount either in cave or technically? What are the limitations where it does become no longer the right tool for the job and BM becomes more appropriate? I know people do SM with at least 4 bottles, but I'm unsure about aluminum-vs-steel for the bottom mix and if people are commonly using large tanks like hp130s with SM config. I also don't know what people are actually pushing the limits of the config to. 7 bottles mounted SM certainly seems silly, but I don't know if that's just due to my limited exposure. So, what is the actual practical limit of what divers are really doing SM?
It depends on what SM system you've got and how it interacts with the rest of the configuration, but available lift is the major limitation for most SM systems.
The Nomad wing, for example, has gotten bigger (Trek Wing to Rec Wing to the current 60 pound Nomad wing) as the Nomad evolved to cover a wider range of SM situations. The downside is that it's gained some bulk and even when you strip off the shoulder pads, back pad, etc, the larger wing makes it notably bulkier than an Armadillo or a Bogarthian style harness with a small lift bladder. So there's a tradeoff involved - smaller profile versus greater lift and what would be ideal in small passage may not work all that well with multiple stages.
It's important to note that side mount wings are also constricted to some extent by the need to control the air cell. A "60 pound" Nomad won't lift 60 pounds as the wing is constricted by the perimeter control of the wing (bungees underneath and the attachment to the waist strap) as well as tank bungees over the top of the wing if you use a loop bungee running over the wing.
I've noted with my Nomad that in it's current old school/bungees over the top and snugly bungeed wing configuration that if I am diving wet (a 5mm suit that's long since been crushed to about 3mm) the Nomad wing works hard to lift a pair of cave filled LP 95s and a stage along with a can light and a primary reel and a couple jump reels.
However, at the same time, if I loosen the bungees controlling the air cell to let a little more gas in the front of the wing, and allow a little more play on the ends of the wing by adjusting the attachment at the waist band, it will easily lift a pair of LP 95s, a pair of stages and a deco bottle with the same wet suit. If I am diving cold water with a drysuit and thick underwear, the Nomad wing then has adequate lift for LP 95s, a pair of bottom clipped stage, a pair of top clipped stages and a deco bottle, and I think it's a lot easier to manage them in SM than in BM - and it's a LOT cleaner and more streamlined in the water.
But, I also see the advantage of a smaller system over all and before I go to MX again, or anywhere else where AL 80s are all you dive, I see a Razor style harness and wing in my future as it offers more than enough for a pair of AL 80s and an AL 80 stage that is adequate for a 2 1/2 to 3 hour dive in the shallow caves we dive there.
So I don't think any single SM system is optimal across the entire range of what a SM diver can do. Still, I don't see my self going back to back mount - just like I don't see myself going below 240' on OC.