Evolve or Level Up?

Always better to evolve first then level up.

Short answer is: it doesn't matter.

Levelling up doesn't influence evolution. However, it makes no sense to power up a pokemon that you are not going to use. It's species and moves determine its usefulness. So the good practise is evolving first so you can know its moves and decide whether the pokemon is useful immediately or not (obviously you can change the moves with TMs later on).

Leveling up first won't give you any information or advantage vs. evolving first then leveling up.

Now, if it cost less to level up when unevolved, then there would be an advantage. But there isn't, level up costs are entirely a function of the level. It costs just as much to level up one's perfect Mewtwo or Machamp from 30 to 30.5 as it does to level a horrible IV pidgey from 30 to 30.5. So it makes more sense to evolve and know what you're getting than to power up and then hope for the right moveset (and in the case of Wurmple, the right form).

There are very good points posted above. However, my counter point would be in the cases where you find a research tasks that needs some power ups to complete them, when you don't have enough candies yet to evolve it fully YET, or when you think this particular species might get a C-day exclusive move and have no immediate need for it (no regrets here maxing a hundo Larvitar before it's CD); in those cases there might be no harm in powering up before evolving but it holds no obvious benefit. I do it from time to time but I have the resources to indulge myself in such things

I have powered up a number of pokemon well in advanced of evolving them. While the views expressed above are mostly true the big advantage to evolving later would be in preparation for community days and future evolutions.

I'm at the point now where I generally would only evolve something if I was prepared to max it or if I was trying to get a specific move before any funny business were to happen.