A Metro (Seattle) bus driver can easily earn $100k, but the range is crazily from $15k to $130K/year, assuming the upper end is someone in it for awhile working full time, and the lower end is very part time.

What is the difference in probability of achieving that pay? And difference in opportunity cost (debt, time, and effort)?

Without this information, an explicit comparison is impossible.

it's not, because you're looking at major carriers, which no one with the minimum 1500 hours will get a job at.