assuming the same gear and skill level, a powertech ALWAYS beats a Mercenary.
1v1 tournaments seem to favor arsenal merc in a matchup vs. Pyro PT. Provided you save Jet boost for Grapple.
When the PT uses his CC breaker is a win-win situation for the merc - if he breaks the 4 second stun you can concussion missile him (instant if necessary) and heal yourself to full, if he doesn't then you'll be able to put full heat signature stacks on him for the 5k Heatseeker.
I don't play Pyro Merc so I can't determine how that matchup is.