Spending fiat or Bitcoin is basically the same because at the time they both hold the same value. You could for example spend Bitcoin and replace it with the same amount in fiat if you wanted to.

The key difference is in which type of money you save for the long term.

nullama

Indeed, if you spend less than you make in fiat it's a no brainer, but in bitcoin terms it becomes tricky because you're spending a certain amount of sats, you might not be able to get  back to in the future