MDs and DOs are both physicians. Both learn at least a core medical curriculum, then take exams demonstrating that level of knowledge (USMLE vs COMLEX). Both can apply for residencies for any specialty, and they take the exact same specialty boards in order to practice medicine. Both can go into research or any other branch of medical science, with or without going through a residency.
The core medical curriculum is offered in all. So I honestly feel that what you learn and your exposure to research is highly institution-dependent, and the differences between MD schools can even be greater than between MD and DO schools. (Except that OMT is not offered in most MD schools to my knowledge.) There are some generalizations, like that DO schools tend to be more holistic in philosophy, but even that is institution-dependent.
Unfortunately, historically, and even still in some circumstances/regions/fields, DOs have had a more uphill battle to reach the positions and recognition that they deserve in this MD-dominated medical field. But within the medical field, I feel that the vast majority of MDs and DOs today finally see each other as equal colleagues, as they should.
Certainly a doctor's skill as a clinician or researcher is more dependent on their individual learning, mentorship and experiences that they seek out than anything else!
Which is probably the most important factor for someone going into research: what the affiliated UNIVERSITY is doing. What research can you be involved in during medical school? What can it offer you in strength of opportunities and mentorship?
I would approach it by exploring the difference between SCHOOLS rather than degrees. Figure out who is doing what you want to do, and who offers the most opportunity to explore what you're interested in. Then apply accordingly. Also, I recommend contacting both an MD and a DO in a similar field of research and ask them for their perspective on their experience, and advice as someone coming into the field.