I think it all depends. If you are going at a time when it is quiet, then I would probably go with the roundtrip fares. I went this way because it is Easter week that we are going. I knew, with the entire New England area (if not the whole country) is off that week from school, that everyone would be grabbing the flights. I couldn't take the chance. It turned out that the combined flights, equal out to 224.00, so thats not too shabby for Easter week.
Just reread your question...when you purchase one way tickets, at least this was the case for me and I'm not sure if its because it was the lowest fare, but they are nonrefundable. If I cancel, I have the option to book another flight in the next 12 months, but it was not refundable. So I don't believe you could transfer these tickets into roundtrip if you found a better fare.