J.S.
I love my Britax Marathon. Strapping her in is super easy and easy to get her tighten. If you meant getting the actual seat tightened enough, then I don't think you should have a problem with the Britax either, as I moved mine from car to car a few times and never had trouble. Just "sit" on top of it while you pull the straps tight.
Try to keep him rear-facing for as long as possible. I kept my daughter rear-facing until 21 mos.