One word: coaching. If your prop firm does not offer quality coaching and training, leave! Also, make sure that you don't enter into a "lock-up period" on your capital. Confirm that you can leave whenever you want.
Well real prop firms offer multiple "edges" 1. exchange rate commisions 2. fastest connections to the exchanges anywhere 3. lots and lots of capital to work with 4. training/strategies to trade
Ditto Youngtrader...I don't think I would ever leave the prop game. Exposure to successful traders is also a huge plus.