I love sidemount, dont get me wrong. I learned it quickly without an instructor but with several mentors and lots of dives. But I dont think its for everyone, and now Im down to using SM mostly for solo dives and exploration
when Im diving with BM people, I show up with BM gear. In a class where the instructors job is to task load you, throwing lights out drills and air shares at you every which way, the last thing you want is to be guessing which second stage to donate, remembering when to switch, fumbling for your lights in pockets you cant reach as easily as in BM, or dealing with hoses that are just long enough to drag in the silt (bug antennae SPGs are particularly bad at this, catching silt or lines any chance they get if you arent careful) or just too short to allow you to feel comfortable.
Second stage hoses: Long hose on one, long hose on both? If long hose on one, which side? Are you going to have a left handed 2nd stage or need to use a hose looping around the back of your head? What is your gas planning to make sure you can share safely if you only have a long hose on one side? (Backmount: any number of lengths will work just fine for the backup depending on your build, and the DIR people have picked a standard length that suits most. 7 hose is standard on the right post, its a no brainer.)
Second stages: Are you going to use right angle adaptors, 45* adaptors, swivels, or no adaptor? Are you going to put them on the same bungee, separate bungee, or clip off? If you clip off, how are you going to handle an OOA situation? Are you going to breathe to thirds then switch? Breathe to 6ths, switch, breathe to thirds, switch, breathe another 6th, then turn? Breathe 200 then switch until you hit thirds? If you are in the middle of switching when an OOA happens, which second stage will you hand off? (backmount: dont use adaptors, they are an additional failure point. Put your backup on a bungee and put a clip on your primary. Breathe till thirds then turn, donate the primary. No brainer.)
Drysuit hose: Left or right tank? Shortest hose possible to go directly to the inflator, or are you going to loop it down?
Inflator: Run from the top or the bottom? If from the bottom, how are you going to run the dump valve string? Are you going to use the shortest hose possible to go directly to the inflator, or are you going to loop it down? If you use the shortest hose possible, what are you going to do if you have an issue that leaves your other tank unusable, leaving you without a drysuit inflator? Remember, not all exits are pure ascents. If you route from the bottom, are you able to manually inflate the wing easily?
SPGs: Do you run them out like bug antennae or wrapped up like stage bottles? (backmount: use the DIR length it works for most. Adjust to fit your body height as needed. Clip it off, no problems)
Bungees: What length is best? Will you use chokers? (backmount: leave the bungees in the truck. (sidemount is nice, the bungees hold your fins while you climb out) )
Rear Connection: Carabiners, bolt snaps, butterfly snaps, suicide clips, or black magic? If bolt snaps, are you prepared to deal with a bolt snap that wont open due to sand? Do you use a worm gear or a cam band? (backmount: use bolt snaps on everything)
Lights: Keep them on your shoulders? On the harness like BMers? In your pockets? Can you reach your pockets? Do you clip everything in your pockets off you let it all fall on the ground while you pull out your light? How will that backup mask feel with a boltsnap on the side? (backmount: put them on your harness and you are done. No brainer.)
I do think its easier to put your fins on in the water with sidemount. My first dive in backmount after months of sidemount diving, I thought I would never figure out how to get my fins on in the water. I also think its easier to scooter upside down and do barrel rolls in backmount. VIPs are easier on sidemount tanks. Reaching your pockets is easier in backmount. Shearing off the knob on the Lips while scootering is easier in backmount. Swimming on your side because you breathed one tank for too long is easier in sidemount. Walking on land is easier in backmount. Running on land is easier in backmount. Tripping on land sucks in backmount.