I was just in that position last year, trying to decide what I wanted to. I ended up deciding to go MD only, so take my advice with a grain of salt, I'm kind of biased 🙂
The MD/PhD should really be called a PhD/MD because it is first and foremost dedicated to research. The goal of the MD/PhD is to become a P.I. and devote 80% of your time to research and 20% of the time to practice. So, what I asked myself is whether or not I would be happy with this balance. Of course, there are individuals that choose to reverse the split, with more time in practice or being full-time clinicians (they are considered "failures" but it's their life, who cares?). What you have to ask yourself is where do you see yourself in the future?
I also considered the time commitment: if you are female hoping to start a family, the 15 years required to obtain an MD/PhD is quite daunting. It's a little easier for males.
Getting your full tuition paid for, as well as the expenses of applying (application fees, flights, hotel stays, etc) is a definite bonus. You also get a unique perspective on research: the ultimate combination of the bedside and the bench. Translational research, which combines both of these aspects, would be a perfect field for someone with an MD/PhD, and yet someone with only an MD could also be successful (many are).
What it ultimately came down to for me was the fact that I did not enjoy research enough to want to spend 4-5 years in a lab, I do want to have a family and it will be hard enough handling medical school alone, and lastly, I realized that if I want to do research, I can do it with an MD. There is so much flexibility and I'm sure I'll change my mind. Research is a required component to the fellowship years so I figure I will make up my mind then.
It takes a very unique individual, with an internal drive, persistence, and a true love for research to successfully complete an MD/PhD. Do you have a lot of research experience? What kinds of things are going through your mind?
Oh, if you are more interested in doing research, have you looked into translational programs? They are PhD programs where you take the first two years of medical school with med students and then do your PhD. It allows you to get the "medical perspective" without the extra training and patient experience. I think they have one at Sloan Kettering...
I hope this helps! Good luck with your decision, it's a tough one!