It's a personal preference, but here are a few things to think about...
*You shouldn't be thinking about how you can "see" your baby while you are driving. It's more important that you watch the road and pull over to tend to the baby if she's crying. With the baby seat facing backwards, you can't get a good look at her in the mirror anyway., and no matter where she is placed you shouldn't be trying to reach for her while you're driving.
*In your head, walk yourself through getting both kids out of the car in a parking lot... if you have the baby behind you and get her out first, then you walk around to his side to get him out and do what with the baby? You either have the car seat that you popped out which you then have to set down on the ground (not very safe in a parking lot) or you have her in one arm or a sling while you struggle to get your son out. If he can get out of his seatbelt himself, this isn't as big of an issue.... If you have your son behind you and get him out first, you can walk with him to the other side of the car and teach him to put both hands (or at least one if he's holding something) on the car while you get the baby out. If he is too hyper or doesn;t listen then that's a problem, but that's what I did with my kids.