C.,
My two boys are both in Britax (the Grandparents wanted to purchase a large gift so we asked for one for each). The first one we got was the Roundabout. I used it exclusively with my first child (he was a spring baby and I hated carrying the infant seat). I think it is the easiest to use, easy to install and seems very safe.
Then when baby #2 came, we bought the Marathon and bumped my then nearly 2 year old into that and put the baby into the Roundabout (he was closer to 5 months when we moved him from the infant seat to the Roundabout).
Now, we are expecting #3, and I am going to by the even bigger one (I think it's the Decathalon), and switch them all up again.
I would never purchase anything else. Especially if you have someone in the family (Grandparent especially) who likes to buy big things for baby, it is a great investment in your child's safety, and who can't appreciate that.
J.