I'd suggest plumbing/gasfitting over steamfitter/pipefitter in Alberta if he's considering either. Pipefitting is more niche and your career route is basically "Pipefitter to 'hope you get into foreman/gc/superintendent or qc/inspector/client-side'. There's a lot less opportunity to start your own business outside of being a single person working as a limited company (which basically means you're hoping to get inspector work). Far more likely to get stuck doing fly-in/fly-out or plant work on rotation as a steamfitter/pipefitter too.
Plumber/gasfitters can easily startup work doing residential, commercial boilers, heaters, hvac, etc. and can more easily hire people to work under them (profit off their labor, etc).
At least that's what I've seen as someone who's had a steamfitter/pipefitter ticket.