At some point in playing "Pokémon GO," a trainer would have to decide whether a Pokémon is ready to evolve or to power up first. Evolving or powering up a Pokémon can be a difficult choice that requires careful consideration. It doesn't help that every trainer has multiple Pokémon and it can be tricky for them to pick which ones to evolve or power up.
First of all, every "Pokémon GO" trainer should have a clear grasp on the difference between evolving and powering up a Pokémon. The actions require Candy and Stardust, but a power up increases a Pokémon's Combat Power (CP) and health (HP) and makes it stronger for battle.
Evolving a Pokémon would transform it into its next stage. For instance, Rattata evolves into Raticate and Ekans evolves into Arbok.
According to The Bitbag, "Pokémon GO" trainers shouldn't power up low-level Pokémon such as Pidgey, Caterpie, or Weedle because they won't be successful in Gym battles despite that increased CP level. Keep in mind that power ups are most effective on tough Pokémon that is meant for Gym matches.
Evolving a Pokémon requires careful consideration, too. For example, if your Zubat has already evolved into Golbat then it's pointless to waste Candy or Stardust to evolve your other low CP Zubats. It's better if you just power up the Pokémon's stronger version.
If your "Pokémon GO" style is to battle other trainers, then you should opt for a power up approach than an evolution. In this case, you'd rather spend your Stardust or Candy to have a high CP Growlithe than a low CP Arcanine. This approach works well when battling other trainers or claiming a Gym.
Bustle, however, pointed out that this style has its downsides. Being a gym leader is a short-lived triumph and you'll get dethroned by other trainers more often than not. If you don't like this approach, you can always focus on collecting and augmenting your Pokédex with as many Pokémon as you can catch, evolve them, and worry about powering them up later.
As for which ones to power up, you might want to bypass weaker Pokémon and stay attentive to the stronger ones with more CP. When you feel like it's the right time, evolve your strongest Pokémon and then use Stardust on it. The combined effect of Stardust and evolution can give your Pokémon a power boost.
