ETA: What is the difference between these Britax seats?

[hudsonam]

Sorry for all the questions, but now that my son is getting older, some of these seats are new to me. I used to be very educated in baby products up to a certain point. What is the difference between the Marathon, Decathlon and Boulevard? I definitely want to replace our Roundabout with something with a higher weight limit, but I can't figure out how these seats differ, other than looks.

TIA!

[hudsonam]

I think I found the answer to my own question in an old post. Thanks anyway! :)