What the Pro vs Cons HVAC vs Piping:
For an engineer or designer, probably not much difference; do some calcs, put some lines on a plan.
For a tradesman, as I suspect you are, new piping goes in before the building is covered. HVAC goes in after. Do you like to work outside, or inside?
Which one pays the most?
No idea; I'd guess it depends on how badly your skill is needed on a particular site.
Which one has the least up and down and for stable?
These days, nobody knows. Here in SoFla, there's a steady business in AC replacements in older homes.
Which one is easier start a business?
Here in SoFla, judging by the people we've hired, the barriers to entry are not substantial. Our experience has taught us to work with older companies. Getting to be an older company is clearly not easy.
Which offers easier employment.
I haven't seen a plumber or an AC tech put in an easy day, except for the ones who have just branched out on their own, who will talk the ear off a brass monkey, telling you how hard they work. ... while some minion does the actual work.
Mike Halloran
Pembroke Pines, FL, USA