Well do you know that the 580 is in good working order? With a new GPU you have warranty, buying used is a case of "buyer beware". The 580 (if in good working condition) will definitely outperform the 750Ti. However it's going to use a lot more power. So you need to consider if your PSU is powerful enough and good enough quality to run it.
The power questions we can answer with more information. The condition of the 580 is up to you to answer.