We have the Safe Voyage and are very happy with it. I believe it is similar in size to the Britax Marathon, but the weight limit is 55 lbs. instead of 65 lbs. (from what I remember). My son will reach the height limit well before he is 55 lbs., so I don't think that's a big issue. The Safe Voyage certainly seems to be made by Britax - the body looks the same as our Britax seat and the warranty card is returned directly to Britax. The fabric isn't quite as nice, but for $100 less, I'm not complaining!
I see you're in Upstate NY. If you're near Albany, Babyland has the Safe Voyage in stock (as of a couple weeks ago). I bought mine on-line, from www.albeebaby.com