I can't speak from advanced experience, but I do have an MD, a bioengineering PhD, and am going into EM this coming year. My interests have varied over the years and so has my path. I assure you that if you start a 7-9 year program that you will meander a bit. With an MD, PhD program, you have some room to follow your own path.
As far as money, if you have any interest in that, go directly into engineering and work on the business end of things. If you are interested in money, run screaming from an MD, PhD program. Even MD programs can be a wash compared to some engineering jobs concerning money.
I have been living on a stipend for over a decade, but have a wife and child and house and cars. I enjoy what I do, I follow my interests, and I am getting to do amazing things. I have a lot of possible career routes and can go straight clinical or straight research at any time. I won't, though because I like to find out things no one else knows, while at the same time learning how to handle any situations and make people "not die." That quote is from a hospitalist friend.
If you like academics, research, and can put up or stay on the periphery of inane politics, then an MD, PhD is great. Bioengineering has potential without bound in EM, as there aren't really many MD, PhDs in the field. This could be due to it just being a "clinical" undertaking, or, from a longer perspective, due to it being a very young field. Clinical (private or academic or government), consulting, industry, academics and research (medicine or engineering), start-ups - name it and you will be qualified for it.
Good luck. Decide based on enjoying your life most of the waking hours. Even a dermatologist has to enjoy what they do, otherwise they spend most waking hours working on something they despise.