Dont bother with the 15xx either. The major strongpoints for Ibanez is the solid mid range line with top quality trems, and it's my opinion that the Edge Pro really is a step down stability-wise, due to the removal of the locking trem posts. It's a better design, and with potential, but at the moment I wouldn't bother because it's inferior to the Edge and you cant swap any other trem with it. Edge Pro guitars ONLY work with Edge Pros.
Besides, the 5xx are usually pretty new, and are CHEAP AS HELL!