This is a decision that will require a lot of soul searching. I actually applied MD/PhD and ended up deciding just to do MD and am very happy about the decision.
How much research do you want to do? You can certainly do research with an MD if you want to practice the majority of the time and collaborate. Or even if you want to do research 75% of the time. There are NIH fellowships that pay residents to train as "clinical scientists" in 1-2 years. I know people who've done this, it is great. There are also opportunities for loan repayment with these programs.
MD/PhD will help you secure grants- but this is a road you want to go down if you are CERTAIN you want to spend a significant amount of time doing research. There are of course financial benefits with MSTP programs, but honestly, when I thought about not seeing patients for 6 years from when I was accepted, it wasn't worth it. You will want to kill yourself during the PhD years unless it is something you're super passionate about. If you are- go for it. Don't ever do MSTP for the money.
If you really really love research and want to spend the time learning how to think like a PhD and immersing yourself in academia, proposing a project, and writing a thesis- it is great training. No doubt, but it is usually basic science oriented. If you want to do clinical research, you don't need those skills and training, and MD is sufficient.
Do you want to write grants for the rest of your life? If yes- consider the PhD. If you want to collaborate with other people and maintain a significant life in patient care, don't bother. Seriously, you can do legit clinical (and even basic science- depending on your background) with an MD.
I'm glad I chose MD in the end, I think as much as I love research (which I do!), I love patient care more. I will always be involved in research, but the time/commitment wasn't worth it to me. That being said, this is a super personal decision you must make for yourself.