Should I buy two S19j Pro's for $3,700 USD and mine it for the next few years or just buy $3,700 worth of bitcoin (0.16 BTC) ?
My worry is that mining difficulty will keep going up and I will run out of time being profitable before I reach 0.16 BTC.
If you're not seriously ready to face the difficulties associated with mining then it's better you avoid going into it and not to be consumed by the difficulties along the way where you may have to abandon what you have already spent much time and energy to build. If your guts tells you to buy $3700 worth of bitcoin then it's also a rational decision as you don't have to face hassles in making profits compared to mining. All you get to do is to HODL which may not even get to 250 days before you start making profits from concurrent rise's in bitcoin price.