I don't know if T32 versus MSTP really tells you much. I think those baseline pools are very different. All sorts of random people get thrown on T32 who really have no interest in doing longterm research. I mean, from an institutional standpoint, its nice to have trainees on the dime of the NIH as opposed to the institution so everyone get thrown on a T32. The people who enter MSTP have different life goals and therefore, would be more successful at research because they keep at it longer due to that being there intention all along.
But they are all getting grants way longer than they used to, if at all...