Here is a link to a spot on the Britax site where you can compare the seats.
http://www.britaxusa.com/products/default.aspx
A lot depends on the your DD's size and the size of your vehicle. And since your daughter is young, you will want one that is a convertable. (Can rear face for now and then be turned forward facing later.)
I drive a Ford Flex so space isn't an issue, but if you have a smaller car, some of these seats are pretty big.
My DD is still in her Roundabout at almost 5 years old and really isn't anywhere near outgrowing it. It will most likely expire before she outgrows it. My boys were out of their Roundabouts by age 3 and age 4 so I would really hav been better off with a Marathon or other higher weight limit seat for them from the beginning.
Happy Shopping!