If you are going to be moving the car seat regularly, the Britax Marathon is awesome! It has the LATCH system with seatbelt like releases and catches...worth it! They are the most expensive, but also considered one of the safest and will get you through until booster seat time. We still use it for traveling as it's comfortable and supports her head for sleeping. It's just tight in our car (Galant) for everyday use and she's almost 5 and doing great in a booster.
P.S. - if your boy is that big and lifting his head, he's ready to change car seats. We didn't know that and waited until 6 months, but our DD was lifting her body up as high as the straps would allow for the whole trip b/c she wanted to sit up. She was much happier moving to the Britax. She was also in those percentiles.