The main difference
Push 2 is a computer-connected controller for Ableton Live. Push 3 can also work as a computer-connected controller, but the standalone version can run without the laptop. Push 3 also brings more expressive pad performance, including MPE-capable playing with compatible instruments.
If you always produce at a desk with a computer, Push 2 can still make sense. If you want newer hardware, expressive playing or laptop-free writing and performance, Push 3 is the stronger long-term choice.