It varies from person to person. I'd rather go to the States, but I know people who prefer to stay in Canada. I also don't think there's any reason to believe the MSTP waitlist would move faster than the MD waitlist. It might move faster, but I doubt it would have much to do with the fact that it's an MSTP waitlist. Apparently the Wash U waitlist likely won't move until May. Anyway, from what I have heard, waitlists are chaotic things - they behave differently every year, and there's pretty much no way to predict them. In your case, I guess I would be patient for a little longer, and as the deadlines for your PhD programs approach, contact the various directors again.
Also, whom are you contacting at Wash U? If you email Brian Sullivan, the coordinator, I find he's incredibly helpful and responsive.