I've done some research on this, not got all the answers however:
Phone line & reasonable speed broadband is available for a minimum 12 month contract so this would probably be your best bet.
I'm only going for 8 months so am going with the dongle, possibly taking a "wi-fi for dongle" router with me to enable me to use my phone at the same time as my laptop however this isn't really necessary but it would be if you are sharing 2 or more devices between a family etc.
Check out the Turkcell website, it's hard work but the prices are there, I believe that signal in Fethiye is good, obviously depending on exact location.
Not sure when you're going but I'll be there on the 25th so can update you with dongle prices on the 26th, hopefully!