Which to evolve and power up?

Torchic all the way, because its end evolution actually is quite useful. Note that both of these two are not THE most meta relevant mons, thus you might aswell wanna invest ur dust in another Machamp or Tyranitar or save it.

Hold on to the Torchic, don't evolve it. Blaziken isn't necessary for anything right now as there's nothing where it's best at, and starters seem to be prime candidates for community days. If it gets an exclusive charge move as good as Frenzy Plant later on it would become an amazing attacker.

The Trapinch I would advise you to not invest stardust into. Flygon is sadly totally useless in POGO.

If you want to power up something now, Machamps or Lati counters would be the way to go.

Torchic is more meta-relevant. Though he is outclassed by MaChamp, Haryama, and Breloom as a fighter and FS Flairion and Fire Legendaries with fire; he would be a solid addition to either team not full of those Mon. Flygon has unique typing that may be useful someday; but for now, he is a decent entry.

If you're in need of a fire or fighting attacker, I guess torchic.

Otherwise, unless empty pokedex spots are burning a hole in your pocket, I'd wait, preferably until the next double points event and evolve them as part of a mass evolution under a lucky egg. Conversely, who knows what some future "research" project will ask you to evolve? Maybe Trapinch will be the May community day mon with some exclusive ground-type move?