This right here. This is IM, a "DO friendly field". Having the MD opens doors in every single specialty! Why do people have such a hard time understanding this? Say OP wants to go GI after his IM residency, well only 1/3 of DOs matched that fellowship. An average MD, is still well placed to get into a solid university IM program that will set him up to specialize in whatever he wants. An average DO does not have that opportunity. And I won't even get into the competitive specialties.
The bold is only semi-true. IM is DO friendly if you're looking at mid to low-tier programs or community programs. For top tier and academic programs, IM is not DO friendly at all. It's almost like looking at two separate fields and IM is too broad of a field to simply categorize as a single "specialty", as there is a massive difference between top-tier candidates and everyone else.
As to why someone would choose DO over MD, I'll give my perspective since I turned down MD interview(s) to go DO. 99.9% of the time, I'd say go MD (including OP's situation). There are a few situations which I think can move the decision to the DO side, but it's not just a single factor, for me it was a combination of many things. I had an interview at an MD school when I got my DO acceptance and cancelled the interview after I got the acceptance. There were really 3 major reasons why I chose the DO school.
The first, and probably biggest one for me, was the location of the MD school and my situation with my SO. The nature of my SO's work made it basically impossible for her to find a job in her field where the MD school was located, where the DO school is located in a city with a lot of opportunities for her (and she's got a job she loves now). So my SO's happiness and ability to fulfill her goals played a role in my decision.
Tied into that was cost of attendance and tuition. My current COA is around 55k/year, and SO's income covers most of that expense. So the net expense for us is around 10k/year. The MD school would have cost me on average 85-90k/year, and my wife would have likely only been making around 25-30k. So the net expense would have been around 60k/year. So financially, before interest, it was a 200k difference. Taking interest into account, I had calculated it would have likely been around a 250-300k difference, and that's assuming I paid everything off in the first 5-6 years after residency. I don't want to think about how much I would have owed had I attended that MD school.
The other big thing for me was that I was only looking at one field where DO's are at a very significant disadvantage in the MD match. Other than that, the other fields I was strongly considering weren't a problem for DOs to match into, even at some of the higher tier MD programs. I also think that studying OMM is beneficial for physicians entering those fields (obviously not all aspects, but there is certainly useful knowledge). Even knowing that, turns out I'm now looking at a completely different field than any of the ones I initially wanted, fortunately it's also a pretty DO friendly field.
Going back to OP, I don't think there's any single reason that's good enough to choose a DO school over MD, or one school over any other really. For something as big as DO over MD, it's a combo of personal and financial reasons along with an understanding and acceptance that you're not planning to take certain career routes. I will say that there were several MD schools that I would have attended over any DO school had I gotten in, but I didn't get in. I'll also add that there were several schools (MD and DO) that I withdrew my application from once I was accepted to my current school, and I don't regret withdrawing my app from those schools.