depth strider vs frost walker, which do you put on boots

you can't have both on one pair, the game won't let them combine. depth strider for most people - ocean monuments, rivers, any time you're in water. frost walker is more fun than useful, it freezes the surface under you while you walk and it melts back after. it does stop magma blocks from hurting you though, so a second pair for the nether isn't silly.