I agree with Colleen H! I think that the Marathon will fit just fine, and where you have only one child you can put it in the "middle" seat so that the front seats aren't affected. This means you will have to sit on the seat to put him into his, but really he's safest in the center, away from the doors.
You should go to Babies 'R Us and ask to try and install the Marathon in your car ~ they are very open to letting people do it and I've not only heard of it being done I've actually seen it. They let you take the display to your car and the car seat person puts it in! This does not mean you have to buy from them, and I would recommend buying online where it's cheapest (http://babycheapskate.blogspot.com/2008/05/20-off-britax-...)
I have both the Marathon and Roundabout and have very large kids. My 6 month old required my buying the convertible at 3 mos. due to height and weight, and I chose the Roundabout as Consumer Reports gave it better *infant* crash ratings (I can send you a copy of the report if you'd like) than the Marathon ~ but it was an "Excellent" vs. a "Very Good" (I think). My 3 year old (now at 35 pounds) is in a Marathon and so buying the Roundabout wasn't a concern since I figured by the time he's three, she'll be in a booster and he can have her seat!
Good luck deciding. I know it's a hard decision, especially with it being so expensive--I just went through it myself!
L. :)