My oldest was riding in the Britax Marathon until he was 6, when he became too tall for it (at 50'' and 45lbs) and moved to the Britax Regent. The Boulevard is basically the same seat as the Marathon, it just has the side wings and its just a shade shorter at the top harness height.
In my opinion.... I would get the Boulevard, knowing I would get at least an extra year, if not two years, out of the seat, keeping my child in a 5pt harness for longer.
Another thing you could do it look into the Evenflo Triumph ADVANCE. It will rear face to 35lbs, and forward face to 50lbs, and most children can use this seat up to age 5/6, and it runs around $150. If you go this route, make sure its the ADVANCE, the regular Triumph is very short and only to 40lbs.
EDITED to clarify that the REGENT and the FRONTIER, both made by BRITAX, are FORWARD FACING ONLY SEATS. You cannot use them with a child that needs to be rear facing, and any child under 33lbs needs to be rear facing.
And whoever said 'i would make my child sit in a seat like that after 40lbs'..... well my oldest was 40lbs at his 3yr appointment.... and no way in heck I was putting a 3yr old in a belted booster! I don't care what the box says, the box is trying to sell the product and doesn't have the best safety of my child at heart. My oldest is now 6.5 50lbs and 53'' tall, in the Britax Regent, where he has at least another 2yrs to grow and remain in the 5pt harness.