I was once a miner, and I do mine using my equipments (antminer s5) then I shifted to mining rentals and to online mining sites. In my experience it is good to buy and hold when a coin is new or during ICO. Its easier to grab or hold a coin at a very low price than to mine it which very time-consuming.

But since I am not a pro in mining this is all that I could share, but I dont know the story of the big time miners, maybe for them its a different story.

Well in the meantime if you are mining, do not sell all your mined coins at the same time but instead hold some of them and wait for them to increase in price that way you will earn surely.