We have a Britax Marathon in our larger vehicle and a Britax Roundabout in our other. The Marathon is great for my child and her long torso. She's been happily and safely rear-facing for 23 months in both cars. We need to turn her in the shorter Roundabout.
I recommend spending the extra money for the Marathon unless you have space constraints in your car. The taller shell will last much longer! Many children outgrow other convertible carseats long before they reach the max weight listed. The longer the torso and the lower the harness slots, the faster the seat is outgrown.