an mba isn't a prerequisite for a canadian phd and won't help you get in as much as you'd think. an mba is more practical and will help you land a good job afterwards, a phd is more academic and you'll have to stay in academia afterwards.
if you really want to do a phd, why not do a masters first in your area of interest? if it's finance, do a masters in finance. if it's economics, do a masters in economics. but an mba is vastly different from these degrees as it feeds its graduates directly into the work force about 95% of the time.
Oxford Round 1 - interview invite 10/30/09, admit 12/11/09, ATTENDING 2010
My Road to Oxford and INSEAD http://gmatclub.com/forum/my-road-to-oxford-sbs-and-insead-89413.html#p676253