None of them. They both are scheming idiots. If she HAD to choose one of them, my vote would be for Faendal. He seems nicer, a little more stable and mature (doesn't live with his mother, ok, that's mprobably normal in Skyrim, but anyway...), and btw, he already has the upper hand, spending a lot of time with her in her house (and apparently also has somekind of thing going on with her if you marry her).
My favorite way to do this quest is to get the letter from Sven, then tell Camilla about it. That way I get a grateful Faendal as my follower/archery trainer and can just pretend that he hadn't planned exactly the same scheme (at least, my character doesn't know about it).