I would wait to get your DD out of Marathon till she outgrows..
Only because there are some new seats coming out soon. I understand Britax is coming out with new seat like Regent.. it will be called Frontier and it is 80lb harnessed seat converting to 100lb booster. We have Regents and love them... but they are big. I have a Suburban and have been able to get Regent in Middle and a Marathon on each side. But my car is big, long and wide. If you aren't planning to have more children then getting Regents won't be an issue. WE LOVE ours.. As Julie suggested the Radians are very nice too.
Kelle
Mom to Riley (05-03)
(She's a girl, not a boy)
Used to sit in REGENT
NOW SITS IN FRONTIER
WE LOVE!!!