The profile of the Dionos adds extra torso room compared to the Britax -- the measurements can be deceiving. I have both and I would say my daughter has at least 2-3 inches more in her RXT. That being said, I think the quickness in which a child will outgrow a Britax convertible is some times exaggerated. Extremely tall kids with extremely long torsos will outgrow them before 3, but the vast majority will make it to 3 and beyond. DD2 was 37" at her 2 year checkup (90% if I recall but kinda leggy for a toddler) and she still has 2-3 inches of torso growth at 27 months. She will last longer in her Boulevard than she would in the Sureride, one of the most commonly recommended ERF seats with a 40 inch standing height limit.
Thanks! I agree about the Britax. I have a 36" 2.8 yr old and she should last quite a while longer RF in the Britax. Is it allowed to recline the actual car seatback to give extra legroom?