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Sterling makes pretty awesome ropes. Just bought a new Fusion Nano from them!

i think you'll be happy with either for a workhorse rope. I'd recommend the dry treated Sport out of the two. I don't have experience with either, but I agree that you should have a dry treated rope especially if you're going to be dipping it into the ocean. Don't forget to wash it on occasion!

Maybe other people can offer first hand experience with the Marathon series of ropes?

I never had those ropes, but as a general rule, I personally like the thicker ropes for climbing. Sure its extra weight and all, but its usually more durable and can take more of a heavy beating. And since your dumping a lot of cash out, you want the most bang for your buck, and the rope that will last the longest. If your just getting into climbing, you will surely be beating your rope. My first rope was 9.4 mm and it got trashed in no time.

Just my personal preference, do what you want though, maybe one is better than the other, I didn't do any research on them.

Dry treated is the way to go!

Sterling Marathon 10.1 is very supple, and a good workhorse size.

What are your intentions with the rope?

Top-roping really beats up a rope. If you start doing lead you'll want a ~9mm rope anyway, and won't want to beat up your good rope on the top rope crag. So, get the cheaper one now and don't worry about versatility - you'll end up with two ropes anyway.