@SWiggs The future plans you stated really sound more like near term plans, to me. Your plan carries you through Intro to Tech. What are you FUTURE plans? It sounds like you intend to advance further, into decompression diving, yes?
In your deco diving, is your interest caves, wrecks, or just really long dives at recreational depths in open water?
If you are interested in caves or deeper wrecks, then (just my opinion) the first thing you need to really do is decide whether a CCR is in your future. Do the research on that, so you can make an informed decision. If you decide that a rebreather is in your future at some point, then the sooner you switch over, the better. And if caves or deep wrecks in on your To Do list, then a CCR is probably where you want to be aiming.
Learning to dive a CCR is a LOT like starting completely over from the beginning of diving. If you're going to do it eventually, in order to achieve your long-term goals, then the sooner you switch over, the sooner you can achieve your long-term goals.
That is not to be construed as trying to rush anything. If you long-term goals need a CCR, and you need, say 200 hours on CCR before it is reasonable to start executing on those long-term goals, then the sooner you start, the sooner you'll have those 200 hours.
I did OC tech training up through TDI Trimix. I switched to CCR a bit over a year ago. The training path for that is, in short, MOD1, MOD2, and MOD3. I am currently at the MOD1 level and trying to schedule MOD2 training for as soon as COVID permits. My long-term plans require being at the MOD3 level eventually.
My OC tech training took me a total of almost 3 years, from the start of Intro to Tech to the end of Trimix. Then a 6 month gap until I started CCR training. If I had made the jump to CCR back at the start of my tech training, I would be 3+ years closer to my long-term goals right now...
If CCR is in your future, but you feel like you simply cannot afford to make that jump right now, well, one, it does not have to be as expensive as some people suggest. I bought my CCR used, for about half of what a new one costs. I have a couple of friends who have bought used Hollis Prism2 units in the last 6 months for an average of $3k (needing O2 sensors, a service, and then ready to dive). So, if you can't afford that right now, I would suggest giving real thought to saving your money for a bit, until you can. It does not have to be THAT much more expensive that getting into OC tech. Especially for someone planning to do both doubles and sidemount.
If you decide that CCR is not in your future, well then, I have dived both doubles and sidemount. When people talk about "boat diving with sidemount is just fine", my sense is that most of those people are talking about boat diving from boats that have relatively easy entry/exit and mostly in water that I would consider to be pretty flat. A lot of my diving is in NC where the entry is not as easy and the exit can be a fins-on ladder in 5 foot seas. I went for a week once, with a buddy diving sidemount. In those conditions, sidemount did not look very fun at all. I was quite happy to be in double steels (backmount). But, if you're going to be diving in fairly flat water or caves, then sidemount is not an issue. Some people like to say that SM is slow to gear up and get in. My experience is that once you have done it some and developed some proficiency it is not enough slower to matter. The problem is mostly people who jumped into SM because "it's cool" and they are not proficient and they DO hold everyone else up.
I think SM and BM doubles are both tools that have their place. Which is better just depends on the diver and what they intend to do. That said, personally, as the weight of doubles is not a problem for me, the only time I would personally see a benefit to sidemount (for me) is if my diving were going to involve getting through tight spots where BM doubles would not fit. Which pretty much means caves (for me). I'm not interested in going anywhere inside a wreck that is too tight for BM doubles to fit.
I have no cave training. My understanding is that you have to (or should, anyway) have full cave training and a fair bit of experience in caves before you start trying to go through spots that are too tight for BM doubles. And, well, for ME, I intend to pursue cave training eventually. But, even if I was not already on a CCR, by the time I was doing cave dives that were that "technical" (I guess? as to require sidemount), I would want to be on a CCR anyway. In other words, I would never see myself doing a cave dive on OC that required sidemount. And, in terms of your original post, that means that I would personally not have any reason to say "sidemount is really the best tool for me, for this dive."
I did sidemount for a while, but it was really only because I'm an OW instructor and I like to be well-rounded as a diver and at least have some direct knowledge and experience of all aspects of scuba that I can get.
Lastly, if you decide to push ahead with BM or SM, I would guess that you should go in with the realization that whatever you think you're going to do next could totally change after you get down the road a ways with whatever you start with. I.e. there is a reasonable chance that if you start with one, your plan to do it for a while and then switch to the other could totally change before you ever actually start with the other option.
ps. I agree with someone earlier. If you don't need to fit through restrictions that preclude backmount, then not having back mounted cylinders is just wasting space.