Welcome to Flyertalk, falcao. If a ticket with a set return date is significantly cheaper (cheaper then the change fee that the airline charges) then it might be better to go with a set date and just change it if needed. However, the downside of this method is if you won't know until the last minute when you need to return, the fare difference that you'll have to pay when changing your date might eat up any cost savings that you might have had since last-minute fares tend to be quite high.

Do you have a particular airline that you're planning to fly with? That might help folks give you more specific advice.