It's two completely different things, like having a regular job and doing some occasional additional work. Mining gives you regular income (though you have to invest in it, and also regularly check what to mine, how to adapt, improve equipment), and bounties pay you some additional tokens that often aren't worth much but sometimes you can hit a bingo, for doing some simple work that usually doesn't involve plenty of involvement (though may involve even more than mining actually) and certainly doesn't involve any investment.