Well, one other consideration is that you see the back side of the faucet, which is either a good thing or a bad thing, depending. Certainly it's a great incentive to keep it seriously clean, but also you may run the risk of scratching the mirror if you have to deal with things like mineral build-up back there, and even with caulking if water stands there very much I would imagine there's some risk of the bottom edge starting to desilver over time.

If kids are using the sink on a regular basis I would opt for a backsplash. More important for me for positioning the mirror is the overall space it sits in. How far are the ceiling and side walls (if applicable) from the frame of the mirror? Centering on the space most important in my book. If ceiling is close to top of mirror I would leave the same amount of space between the ceiling and the top edge of the frame as the bottom edge of the frame to the top edge of the backsplash.

I vote backsplash--it protects the wall in case of splashing & splashing is bound to happen at some point with the sink. Better safe than sorry. We just had ours installed in our new home build & we staged it first & found that 3/4 of an inch between the top of the 4" backsplash & the bottom edge of the mirror frame worked best with the size mirrors we decided to go with & the positioning of the light fixtures. I definitely recommend staging first with the dimensions of the items you have chosen no matter if you do the backplash or not. Hope this helps!

thanks! -- we decided to go with the 4-inch backsplash