Power up or Evolve?

The biggest problem was that the moves after evolve are random, and you could have a very good pokemon with a useless combination. Now, with MTs, you can change it.

you don´t know if the attacks it gets are good you don´t want to waste stardust on something you don´t want or need

At Level 21 you probably shouldn't be powering up much of anything (unless it's a high IV TTar or Dragonite). As you get higher in level you'll be able to catch higher level pokemons which in theory would require no dust.

The thought behind evolve before power-up is old information before the game had TM's. Before raids existed you couldn't change movesets, some move sets were so bad it wasn't worth powering up even high IV versions. But now that you can "fix" movesets, that concept is obsolete (unless you cannot acquire TMs).

In the main series games, choosing to keep your pokemon unevolved allows it to learn its moves at lower levels than its evolved counterpart, so you have a strategic balance between stronger moves earlier or stronger stats now. Ultimately, it doesn't have a long term effect on a pokemon's stats when you allow the pokemon to evolve, but there are occasionally a few different moves that can be learned.

In Pokemon Go, powering up then evolving or evolving then powering up makes no difference for the pokemon's stats. You can't improve a pokemon more by doing one way over the other. Moves are randomized when evolving though, so even with TMs it is still generally a good idea to evolve first if it is something that you do not want to waste stardust or TMs on if it gets a bad moveset.