buying coins and holding is capital gain passive investment income.
buying a miner is self employment income it is active.
so for USA investing in coin vs mining with coin involves different tax laws.
I realize neither one will earn fiat money if btc drops in fiat price.
But if btc rises in fiat price both will profit and if you are USA based you will have different tax laws to follow.
US tax law makes it extremely undesirable to reinvest BTC into mining hardware. If you buy gear with BTC you need to pay the short term capital gains on your BTC. That is an extra 20-45% that your mining hardware has to earn before you have a profit on the purchase. Plus you can only depreciate the hardware cost over 5-7 years, so most of the earning from the new equipment will be taxed as ordinary income.
Since pricing of hardware barely supports break even as it is - a US miner reinvesting BTC is literally throwing BTC away by purchasing today.