How does each specifically relate to growth? Is it two different ways to measure the same thing or do they work in different ways?

Thes explanation I've ever heard for this is that potential is like an empty glass. The more potential the guy has the bigger the glass. We is how fast the glass will fill up with water.

If if a player has high potential but low we he will continue improving just at a slower rate, until the glass is full. If a player is low potential with high we his glass will fill up quicker but once it's full he will improve very slowly.

A player improves as far as his potential will let him. His we will determine how fast he gets to that point.

I think potential also plays into how much you improve during the season too. For instance, a good potential player will improve more than a medium potential player with equal WE, same with STL and Good, etc. Looking over GUESS ratings, I have noted this difference for a few of my players.