It helps a great deal to know your child's age/weight/height + vehicle year/make/model
Generally, unless you have an extremely small backseat then it's a better buy to choose the
Marathon over the
Roundabout as the
Marathon offers higher weight & height limit in the harness so you can be sure kiddo will really be mature enough for a
booster when the harness is outgrown....
Evenflo Triumph: this seat has nice low harness slots for newborns, but also an extremely short shell (frame) so most kids outgrow this seat long before they're truly ready for a
booster. Evenflo is upgrading this to add another top harness slot + more height to the shell! I think I remember this would be available in the summer?