D.S.
My daughter is in a Frontier (she is 5), and my son is now in her old Marathon (he is almost 3). In my husband's car we have Regents but those don't fit in my car. I heard that the Frontier is replacing the Regent model because it has the extra safety of side head rests. So I would suggest the Frontier (or the Graco Nautilus, which a lot of people also seem to like).
The salespeople at Great Beginnings in Germantown are very knowledgeable about the different seats and I am lucky that they steered me away from the seat I was going to get (a Radian), which they said she would grow out of too quickly. I think the Frontier will last us several more years.