The Flex line is an entry level variant to the Free line. Good shoe but, IMO, materials and feel, comfort, etc are better with the Frees. Of course everyones foot is different so I encourage you to try both on.
BTW. Little known fact for all of you: Nike has a 30 day policy on all their shoes... doesnt matter if theyre worn to death or worn once, return them with receipt before 30 days with no questions. Only shoe company i know that does that. Literally can wear them out of the store and for 29 straight days of hell...and return them. No questions asked. As far as I know thats still in place.