Britax is suppose to be releasing a combination seat (harness then booster) sometime soon (hopefully within the next month or two). It sounds like a pretty good combination seat with high harness heights, but no one has actually played with it so it is difficult to recommend a seat that you haven't tried. If you can wait a little while, it might be good to wait and see what this new Britax seat is all about.
If you need a combination seat now though, here are the only seats that convert to a booster (note: your marathon has about a 17 inch top harness height so that should give you an idea of how these other seats might work for your child):
Graco Nautilus (~$149) - top harness height of 18.5 inches. Can hold children up to 65 pounds. This seat is forward facing only and converts from a harnessed seat to a highback booster to a lowback booster.
Cosco Apex 65 (~$80-$129 depending on model) (also referenced as Safety First Apex, Safety First Biltmore and a few other names) - top harness height is at 17.5 inches and can harness from 22-65 pounds. After the child outgrows the harness, the seat does convert to a highback booster. This seat does require a headrest behind the seat so it will not work in cars without headrests or tall seatbacks.
Where is your child's shoulders in relation to the slots on the marathon? That should tell us a rough estimate of torso height and might allow us to suggest some other seats that might be a better fit for you. Also, how much does he weight? Sometimes a harnessed only seat might be a better fit for you then one that converts to a booster.