Neither. And honestly, you shouldn't compare them. The only similarity is that video games are involved. Gamestop rips you off sells you video games, and Game Fly is a game rental site.
'Renting' games doesn't really suit my gaming style. If I'm going to beat a game, I'm going to own that game so I can play it again. In my eyes, Gamefly is a casual gamer system. In the old days when games were shorter, I could understand renting some games to try them out. And with the longer games like RPGs you could try them out and then if you liked them, buy them at the store (like BlockBuster) when you went to return it. Renting games through the mail is really, really weird to me.
And Gamestop is the worst gaming store I have ever had experience with. The employees are either jerks who only half pay attention to what I'm saying or they're guys who are over excited to see a female organism walk into their store and know something about video games. The employees always ask if you want to get an subscription to that stupid magazine, ask you to get their game card thing, and they rip you off when you go to sell your used games. And their used games are way too expensive. Not to mention the fact that they open their 'new' games and take things out of them.
Once I was in a game stop and an elderly couple were trying to buy video games for their grandchild's birthday. The three employees that were in there were hovering around me asking questions and if I needed anything. They would not leave me alone until I snapped at them. I had to help the couple myself.
I've never had a good Game Stop experience. And I've visited at least 5 of them.
If I'm going to buy a game, I shop at Wal-Mart or off Amazon, thanks. I will never do business with Game Stop anymore. I miss BlockBuster.