I understand the confusion! Once you have a RFing toddler, you start to notice leg room, but it's all about kid comfort and that's very hard to predict.
It's really hard to tell if your kid will get to 2 in a Marathon, but if he's 'regular' sized, with 'normal' proportions, I'd say yes. I don't have personal experience with the ProRide, but they're pretty comparable shells if I recall correctly.
Now a Marathon doesn't fit little babies well, so you might, right now, be right on the edge of fitting in it. If you buy a Marathon be sure not to use it until the straps can be at or below the kiddo's shoulders.
Britaxes are easy peasy. If you want easy peasy now, go for it. Deal with the later later. You're already ahead of the safety game if you know that RFing longer is the safest way to go. Gold star.