Needing some advice update

Just wanted to give an update on my car seat searching and seek more input.
Julie so kindly responded, so I am curious what she thinks. I went and bought the Graco Ultra Cargo and then this weekend went to a car seat check event. While the technicians are obviously not allowed to formally make specific recommendations, I was able to read between the lines and the take home message was that my kid fit, but not very well, in the Ultra Cargo (they said things like, "this is okay" where I would like to hear something like, "this is good, your kid will be safe in here") and it was pretty clear that they were pushing the Marathon. He is 3, 32 lbs and 36 inches tall.
Here's my question: What is the height maximum in the Marathon? Is it heigher than in the Roundabout? We just moved the harness to the top slot in the RA. I don't want to spend $250 if he is going to outgrow it in height in a year or so....i know every kid is different but in general, how long are these Marathons lasting?

I am beginning to think that maybe I should put his Britax RA in the second car and get a Marathon for the primary......