In One Last Time by Gary Soto what are two similarities and two differences between picking grapes and chopping cotton?

What is the central idea in this advice from branch rickey to jackie robinson?  

Abelief that one's culture is superior to another.

Ineed simplifying this thesis and making it easier to understand for the reader (more concise) "no matter how religious one may be, one’s faith tends to change under unexpected and challenging circumstances."