I'd do MSTP if i could, but they ain't going to take me (let alone a standard MD program) with my stats..haha. So I'm doing a PhD right now. Based on my current assessment, I should finish in a total of 3 years, but definately within 4 years.
How long it takes to do a PhD, regardless of MSTP or standard PhD depends on your program, and research. For the most part, MSTP's tend to finish their PhD in 4 years, thats why programs usually say its a 8 year program, however other programs may give a range of 7-8 years. Seven years is possible because some students are able to waive some or most of their PhD curriculum with their med school courses. Others are able to accelerate the MSTP by doing research while being an MS1/MS2.
Your project will also be a big factor too. The time it takes to do a thesis does not neccessarily correlate with how "good" a thesis is. I forget if an MSTP is bound to finishing their PhD within 4 years so someone please clarify that. But for PhD's, I think the really encourage you to finish within 4-5 years. But people have gone over that range.
So in my opinion, if someone wants to get both degrees, I'd go with the MSTP since you're at least in the MD program already, so you don't have to deal with the application process
. Of course as PhillyMD2006 says, you your tuition paid for and an monthly stipend
. However its amazingly hard to get into. The dean of admissions at UCSF called MSTP's: "the people that walk on water". I don't think he was implying that these people were gods, but more along the lines of these people are really amazing
. If i recall, the 12 or something people that get into the MSTP at UCSF had an average MCAT score of 34, and GPA of 3.85. Plus all of them have some large amount of previous research experience. If that is what you want to do, then go for it. On a side note, I think its possible to get into an MD program, then apply into a PhD program AFTER. However you will have to pay for med school. THat is another alternative.