Hi - I have the Marathon, and I believe it goes up to 65 (or is it 80?) pounds. Anyway, my son, who will turn 5 in May, is barely fitting into it now......and he's not a fat or especially tall kid! I think he currently weighs about 50 pounds (53 max!). Before I suggest you go with the biggest Britex you can get, you need to consider a few things...
1. An older kid (5 and older) may not like the 5-point harness. Yeah, it's the most safe, but kids get a mind of their own at a certain age, and you'll have to be the one to listen to all their griping if they HATE their carseat.
2. Booster seats work with your cars seatbelts, which is much quicker than those 5 point things. Also, the kid can learn to buckle themselves in/out, which can be a huge time saver for mom (especially if you work, and you're the one that drags your kid to daycare/school each and every day).
3. I think a quality booster (one with some sort of head support so that when your kid falls asleep in the seat, their head will remain relatively straight, rather than flopping over sideways, which looks majorly uncomfortable!!!) is the way to go, once an averaged sized kid turns 5.
All in all, I have LOVED my Marathon, and so has my son. But I'm a little disappointed in the fact that there is NO WAY he can stay in that seat until he reaches 65 (or whatever that number is) pounds! I am cramming him into it now!!!
Hope that helps. Good luck!