The Roundabout will rear face to 33lbs, forward face to 40lbs. Most kids outgrow this seat between age 3 and 4. Its a good seat, but for that height/weight limit, you could spend $100 on another brand and get the same use out of it. Not worth the extra money for a Roundabout, IMO.
The Marathon will rear face to 35lbs, and forward to 65lbs. Most kids will outgrow it before they ever reach 65lbs, however most kids can easily use this seat to age 6. My oldest is 6.5, 50lbs, and 52'' tall and just outgrew the Marathon not long ago. The Marathon is the seat Iwould go with. In February, Britax has its big sale, you can get a good deal on a Marathon (usually $50 or more off the price) online. albeebaby.com or tinyride.com has good deals.
I would NOT recommend the Regent like hte other poster suggested. My youngest is 3 and the Regent is STILL big on him. A 1yr old would swim in it, and its not safe to fowrad face a child that soon anyway. I know the 'norm' is 1yr and 20lbs, however the AAP recommends children remain rear facing until at least age 2 and 33lbs, or until the rear facing weight limits of the car seat. The Britax Marathon will rear face to 35lbs.... your child should remain rear facing thatl ong. My boys were rear facing to 19 and 22 mos. Injury sustained from a child forward facing too soon (before age 2) even in a minor collision is fatal. Please research extended rear facing before you turn your child forward facing. He should remain rear facing for at least another year. Its not a milestone.... please consider extended rear facing.
ANYWAY.... the Marathon would be your best bet. Its a great seat, comfy and easy to use, install, clean. THe harenss will not twist, your son will be able to easily sleep in it. It will easily hold him in hte harness until he is 5/6yrs old, then you can move him to a simple booster until he is 8yrs, 80lbs, 4'9'' tall (the recs for sitting in the seat wihtout a booster).
ETA I just want to clarify, someone said carseats expire after 5yrs. Its 6yrs from the date of manufacture, unless you get a SUnshine Kids Radian 65 or 80, they do not expire until 8yrs after the date of manufacture. A Marathon will expire 6yrs from date of manufacture, and you child will easily be able to use it until he is 5/6yrs old, so you will get the full life out of it, then can move him to a simple booster at 6yrs old.