Like RCFProd says, get the 7 or 8.1. More stable for now and more cheaer than win 10 cause you dont need to upgrade to 10 now. Upgrade when it will be stable.
At one point Microsoft said you had to activate windows 7 or 8 before July 29 to get the free upgrade. I would double check on this. They most likely have changed the statement by release though.