Please, please, please, for everyone's sake, no matter what class make sure the baby is sucking on something on takeoff and landing. Both are murder on children's ears. It's easier to try to prevent the pressure changes from hurting their ears than to deal with them when they are in pain. (Not that it always works. If the kiddo is coming down with an ear infection nothing will help.)

I'd opt for first myself. More room. I sure wouldn't get a seat for the first few months. Sure, the kiddo may become a projectile, but kiddo is not going to be in his/her seat all the time and you can't know ahead of time when turbulence will hit. I think once the child is too heavy, mobile to hold for a long flight (hmmm, I'd guess six months) then I'd opt for the extra seat. In that case, you might want to go coach, and have the whole row to yourselves.

I'm a mom, BTW whose son started traveling at nine months (to Jamaica, no less).