Everybody has their own personal limits on things. Sometimes it's price. Sometimes it's a feature. Sometimes it's a brand. It's kind of like buying a car. They all "go" but how will you get there? My car has econo seats so we like our van for longer trips. Nothing "wrong" per se with my car's seats but the van is more high-end and more comfy.
All that said, there were some features I liked in the Marathon that put it top of my list. The rear-facing tether, for example. The high weight limit for a 5pt harness. The ability to easily adjust the straps. The ability to easily wash the cover. The satisfaction of many of my friends. Other friends and family have their reasons for Radian or Graco or whomever.
Bottom line is you need to be happy with the seat you'll likely have for 6 years. Your kid needs to be comfortable in it. All are technically safe. So if you can't afford a more expensive seat, get the best fit for your budget, kid and car vs going to a thrift store and getting a seat you don't know the history of. NEVER buy or accept a mystery seat.
If you are a member of Consumer Reports (or someone you know is), they've rated seats. http://web.consumerreports.org/carseats/index.html?EXTKEY...
I edited out the second site b/c they had some weird criteria and listed the Marathon as FF and min weight of 20 lbs. That might be the limit to BE FF but you can RF in a Marathon now to 40lbs. High rear facing weight was also one of the things I personally looked for. Ours only goes to 35, though.