My viewpoint:
Plumbing is more labor intensive, more digging holes, more fighting with 75 year old cast iron pipe in inaccessible locations. Plumbers typically get dirtier than HVAC techs. septic tanks, sewer lines, muddy trenches. On a work site, plumbers pinch their sandwiches by the corner and throw the corner out without eating it. Plumbers need to know the code and need to know lots of construction trades so I don't mean to imply that plumbing isn't a smart man's trade but it does require less complex thinking and troubleshooting ability than HVAC most of the time.
But HVAC has it's down sides too. You need to know more electrical than an plumber, in fact more than some electricians since control circuits are a whole field of their own. Plumbers rarely spend an hour standing on a flat black roof in the sun on a 100º day.
I prefer HVAC work. It just seems that most of the time, with HVAC there is a problem and it can be fixed. Whenever I get involved in a plumbing problem, it always leads to a dozen other plumbing problems and I get frustrated.
Check out a plumbing discussion forum. Not as lively discussion as here, rarely is there a thought provoking work question asked, and the crowd isn't as helpful. I think that's typical of the two trades too.
Ryan
Maintenance Guy
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naysayer, skeptic, conspiracy theorist