I have a 7 month old DD who is approx 17lbs and is quickly outgrowing her RF infant bucket seat. We are looking to move her to a RF seat around 9 months. We also have a 2 year old DS who is in a FF Britax Marathon. We drive a 2007 Dodge Caravan so space is not a concern- the kids seats are individual seats as opposed to the bench seat. My question is... should we move DD into DS Britax and purchase DS a Britax Frontier? Or should we purchase another Britax Marathon for DD? What would be the best solution here attempting to be cost effective in our choices?