It really depends on what you want to do?
I too want a bow mounted unit, although Im going to take my Humminbird P-Max dual cone 220 and move it up front and put the better unit in back. Saving the coin for a Humminbird Side Imaging unit. Granted that unit would be worth more than the rest of my boat. But if your serious about fishing, at least catching fish, you have to be where they are.

The disadvantage of using the same unit mounted in the back is will you be able to see it from the front?
Will you be able to navigate forward while looking backward?