Getting "buy-in" from athletes: Is it a request? Or a demand?

Request buy-in, demand their best.

In your experience what’s more effective for teams?

Some things are requests. Others are demands. It is up to each coach. Just don’t confuse the two. And know how you will respond when someone declines your request or your demand, directly or indirectly.

Request but the players need to be convinced (through coaching) that this is the only way to be successful. If they don't believe that themselves then they are only robots submitting to your demand as a coach making it less effective.