i have never owned a sub or a van but i have had lots of experience with them (family). although i'm a stickler for safety and would overspend my budget to satisfy my carseat addiction (LOL) and i would also go out of my way convenience-wise to have the perfect seating configuration, i don't think i'd go so far as to have two center seatings in a sub. getting to the 3rd row in a sub is an entirely different thing than doing it in a minivan!
if i were in your situation i would probably keep them in the center row and put one outboard.
when my kids were in a combination and infant seats, i had the infant seat on the outside. i was a little leery of having the tiny baby on the outside but they do lay back into those side wings a lot better than an older toddler does. later on when i had my baby in a rearfacing convertible seat, he stayed on the outside because his conv seat still had deeper sidewings than my older toddler's combo seat.
now that i have a husky (HUGE seat) for my older child, they both ride outboard because there isn't room for the two seats to be side by side anymore.
there is a school of thought that the sidewing advantage is so small as to not make any difference. but from a common sense perspective and some knowledge of crash dynamics, it does seem that a reafacing seat would have a slight advantage over the forward facing one, again, especially in a small baby who lays back in the sidewings. of course, it all depends on chance and the exact point of impact in a crash.
some parents find that they cannot have their children side by side at all because they tend to pick on each other. so in that situation they both end up outboard.
i have an 18 month old and a 3.5 yo and if they were side by side now, the baby would pick on my older toddler ALL the time LOL instead of the other way around.
congrats on the little one coming! siblings are a true joy.