Britax Carseats

The britax seats are fantastic. I really don't think you can go wrong. I also love the Marathon and think it's a perfect seat. You never have to worry about them outgrowing it before they are ready for a booster.

Well we always had britax for our older boys but for 16 months old we got Diono Radian xrt
http://www.dionocarseats.com/s-193-radian-rxt.aspx
it's amazing!!!!!!! It actually has a steal frame on the inside that none of the other car seats have.
Our oldest (7 y o) is in the Britax Fronteir 85 just because he is too big for the Diono.

we have the boulevard and I love it. The only difference between this one and the marathon is the true side impact. I went back and forth between these two and decided that the extra protection was worth it. There is the boulevard CS, which is the next one above the one I have...all it is...it clicks when the straps are tight enough. We are looking at the roundabout for a second one to have in my husband's car...anyone I know that has it likes it...they say it's great for a 2nd car seat or a smaller car. the weight limit is less than the others though. hope this helps! good luck!

I have a Britax Decathalon in my husband's car and a Sunshine Kids Radian XTSL (5-80 lbs) in mine. Love both, but I prefer the Radian as she has more space and it's easier to get her in/out as it sits lower and not as contoured, and most importantly, rear facing to 45 lbs. I do find the Britax a bit easier to adjust. There is also an updated Sunshine Kids Diono Radian RXT model that goes from 5 to 120 lbs and is up to 45 rear-facing.

If the marathon your son used isn't expired you can use that for we and get him the britax frontier, that's what we have done. And now that our 7 year old can be in a highback booster the younger one is in the frontier.

We love the boulevard and would get another if we have a second child