Welcome to FT Surf40

As gengar writes, this is an very individual decision. partially based on what else you would plan to do with the $$ or miles, which you feel you have the most/extra?

A general rule many use for miles is to get >1cpm (cents per mile), targeting closer to 2cpms. Using miles for upgrades or premium class award seats can equate to much more than 2 cpm but those may not work for everyone.

UA sells miles to individuals for 3 cpms (but many think that is too pricey).

But if you have the miles and no better use for them, than it saves $$$