MD/PhD students are more highly competitive than straight MD students
when it comes to residency programs.  Although all residencies will
see it as an advantage, the degree to which they view the added PhD as
an advantage is program-dependent.  Some more procedure based
specialties, such as surgery, may not regard it as highly as other
specialties, however it is ALWAYS helpful.

As an MD/PhD student, I can tell you that you should not go through
this type of program unless you have a real love of research.
Additionally, to gain admission to any reputable MD/PhD program you
will need significant past research experience (as well as an
excellent academic record).  If you are using it as a means to become
more competitive for a medical residency, then I highly suggest you
find other ways to do so.

Rather than discussing this with a faculty advisor at your school, I
suggest you directly contact a number of MD/PhD programs to ask
questions.  Very few faculty advisors at undergrad institutions are
knowledgeable about these programs.