I can help you a tad.. Personally I wouldn't go with a Roundabout just b/c it has the smallest weight limit. It looks like your little one is just 4mo? So you're looking for a convertible for after the infant seat? If so your options are the Roundabout, Marathon, Decathalon, & Boulevard . The Frontier is more of a booster seat w/ 5pt harness & doesn't start til 2yo & 25lbs.. Ihave both the marathon & the decathalon seats.. They have the same weight limits ect (I do love that Britax supports extended rear facing with their weight limits!) but the Decathalon has some extra padding .. If buying for an infant, I'd choose the Decathalon ..
Good luck 
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Proud Mommy to 6 munchkins:
13, 11, 5, 4, 3, 1
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