I am a certified Child Passenger Safety Technician (car seat tech). The safEST place in the back seat is the center. You would need to use the seat belt for install in that location. However, sometimes an outboard install is necessary either because the center seat belt install isn't good (some seats and some cars don't 'play nice' with each other) or because you need to put another seat or passenger on one side or the other and in some vehicles this is problematic. Rest assured that while center is safEST, outboard is still incredibly safe statistically for a child properly restrained in a rear-facing car seat that is properly installed.
If you choose outboard, there is no statistically difference in safety between driver and passenger side in terms of crashes. However, if you do street parking often, I'd recommend passenger side as the child is always being removed on the sidewalk side of the vehicle then...which would be safer. I think this becomes even more important when you're no longer using an infant carrier and have a toddler who can walk (read: bolt from the car).
I also strongly recommend you find a certified CPST who can check your install. If you need help locating one, please let me know! And a great place for parents to visit is car-seat.org, it's the 'Dis' for car seats!
Good luck with your new baby!