As far as I understand it, the chances are the same. I wish I remembered the numbers now... I think that B/D accepts 10% of those who apply, which is the same ratio as straight D.
Here are some websites:
<a href="http://biomed.brown.edu/Medicine/Admissions/Admissions.html#Brown/Dart" target="_blank">http://biomed.brown.edu/Medicine/Admissions/Admissions.html#Brown/Dart</a>
<a href="http://www.dartmouth.edu/dms/admissions/brown.shtml" target="_blank">http://www.dartmouth.edu/dms/admissions/brown.shtml</a>
I chose B/D because I thought that studying at D for 2 years (isloated, beautiful, fun and bright students) would be excellent, but that I would want a more urban clinical experience during years 3 and 4. (However, I just withdrew my application, so this is a moot point.)
Also, students at D explained that every year some B/D students want to switch to straight D and some D want to switch to B/D--while this is frowned upon by the administration, it is possible to switch as long as someone will switch with you.
Good luck either way!
mma