I do not know how to answer you because I don't know how big your car seats are, however, I would say that the more and the bigger your stuff .....on an airplane, the bigger pain it is!!
Less is definitely better.... there's not much room to get them up and down the aisle, especially when you are dealing with the kids too. One suggestion I heard is to have one parent go on the plane with the seats and get them buckled in, while the other parent hangs out in the seating area for a few minutes. This can clear up a little bit of the congestion.