We've just gotten back from a trip to the US with DS who is 18 months. We didn't buy a seat for him on the plane, but were able to get the bulk head with the bassinet on the long flights. I think because your LO will be so small you'd be fine with the bassinet and no seat for them. We rented cars in CA and GA and both times the car seats were fine, clean, good quality and easy to fit. I think if you go with one of the big name car hire co's you're pretty much assured of getting a good quality seat, so personally I wouldn't bother taking one with you and buying a plane seat for your LO. You end up having so much luggage when you travel with a baby that having to also lug around a car seat is just an extra hassle you don't need iykwim.