Everyone does not have the budget required to buy mining equipment. There are also a number of monthly expense involved like electricity bills and thus investing is better in my opinion.
Those two things are good ways to earn bitcoin but it depends upon your capability which is better. Having extra money makes it easy for you to just buy bitcoin and make it grow. On the other hand it's also good to just mine and wait until it grows. Disadvantage of them both is that buying needs your investment while mining needs your patience and also you have to used electronic gadgets and electricity. But for me I will choose to buy rather than mine.