I am a sidemount tech diver who dives primarily off boats. So:
BM doubles: you already know the pros and cons
Sidemount:
Let me echo
@Tracy and stand on my sidemount training soapbox. First, most people can teach themselves sidemount, in the sense that they can get two tanks more or less on their sides and go into the water. But the clearest place where competence on the sidemount platform shows itself is in gearing up and getting in the water. I have seen "sidemount" divers that are just a hot mess and take an hour just to get in the water. That skill level has no place on a boat. A competent sidemount diver takes no more time to gear up and get in than a BM doubles diver, and that's where you need to get to before you take your rig on a boat. Training, system, kit optimization, muscle memory, and practice.
If you want to be a sidemount boat diver, do yourself a favor and learn from someone who is notably competent and enjoys doing that kind of diving. There's no point trying to learn sidemount boat diving from an instructor who doesn't prefer sidemount boat diving. It's different than cave sidemount. Cave emphasizes streamlining, boat sidemount prizes fast gear-up and go.
With that out of the way, sidemount is most helpfully looked at as a flexible platform that can do many things. You can do 2 tanks, tanks + deco bottles, add stages, or monkey dive, which is going with a single tank and is a lot of fun. If you've developed your skills you'll look cool, and you'll look even more cool walking up the ladder with one or no tanks because it's so wonderfully easy to hand them off or clip them to your equipment line. Not to mention how cool your knees feel.
Sidemount is superb for small boats and many 6-packs. If the racks are on the deck, DM doubles are a pain. Not sidemount. Lean down, top clip, put on your bungeed reg then get your mask and fins on and off you go.
If you like playing with your kit, sidemount will give you plenty to play with. If BM doubles is putting on a backpack, a sidemount rig is like putting on a tailored suit. Everything is adjusted to fit you and everything can be modified to suit you. The only thing original on my rig is the wing. You will also develop a love for bungees. Bungee cord is our friend.
Obviously it is easier to take your tank rigging with you and rent single tanks than it is to rent doubles. Opposed valves are very much preferred, but regular valves will work in a pinch. More reaching around, but valve drills remain easy.
I advise you to try it out. If you love it take a class from someone who does what you want to do. If it's not your cup of tea and you aren't feeling the love, well at least you tried it.
