I wouldn't recommend either the roof mounting sort or the front
mounting....why subject your bikes to  bugs, flying rocks, etc. when you
can attach it to the back?  I don't suppose that it is all that important,
but attaching it to the back also isn't going to increase wind resistance
as the other ways will.  Also, why spend lots of dough when a swing by
Academy (or other cheap sporting goods store) can get you a rack for $30.00
(sale)-$50.00 (spendy name brand)?