This is my thought on the topic. Based on your documented progression over the last couple years, I have to assume that you will practice in a controlled environment until you are proficient at single pilot IFR, backcountry flying, or underwater basket weaving. Thus, I have no doubt that you will have an aircraft that is equipped for the task and be trained.
I see the risks of single-pilot IFR as more strategic or operational risks, while backcountry flying as tactical. By that, I mean that flying IFR involves hazards that manifest themselves based on decisions you've made much earlier in the flight. The sequence of events that leads to disaster is a much longer highway with lots of exits that sometimes are hard to identify. For instance, the case study recently published of a Doctor crashing near Dover AFB. He struggled for almost two hours with his equipment and the weather before running out of gas in about the same place he had been two hours prior. Looking farther out for all the factors that will affect the successful or unsuccessful outcome of the flight is essential.
Backcountry flying is more a series of unlinked tactical decisions. You get to the mountains and the winds are too high --> divert away from the mountains. Make a couple passes of the intended area and determine if you can land --> land or don't land. Problems happen on the landing or baulked landing in the tactical environment.
Thus, I think neither is more dangerous than the other as long as you are proficient, equipped correctly (and understand how to use the equipment), and understand where, how, and what the hazards are for the flight. I don't fly long legs of IFR where I'm assured to be in the clouds. That's just too tiring and I'd rather have a beer. I will and have travelled very long distanced single-pilot IFR, even when the destination is IMC/IFR. I damn sure have a plan B and C (similar to GUMP's discussion of backcountry flying I've seen in other threads). The key is to continue to determine what the environment is surrounding all those plans.
Just my take as someone who is much more experienced in IFR/IMC with my single-engine airplane than I am in the backcountry.
Brett