So I am looking at purchasing a new motor but they give me a choice of different rotors. These are all 12.5mm rotors but there are 3 different ones to
Choose from. I'm not even sure what you would call the number, whether it would be a strength rating or what. But they have a -1500, and 1500-1600, and then a 1600+
Any help would be greatly appreciated.

The higher the number, the stronger the magnets are.
That's basically it in a nutshell. When I have the option, I always prefer the strongest rated rotor I can get, seems to work well.

If this is a Fantom motor that you're ordering, just put in the notes section what class you're running and that you'd like them to pick the rotor that would be most ideal for that class. They can also set the timing and give you a pretty good idea of what gearing to run.