An hourly rate transfers all the risk to you: a slow diagnosis costs you money. A flat rate quoted after diagnosis transfers it back. Ask directly on the phone which model the company uses, and ask whether the quote can change once work starts.
Plenty of companies advertise 24/7 service and then charge double for it. The question to ask is simple: is the Sunday night price the same as the Tuesday morning price? Get the answer before you need the service, not after.
For a recurring drain problem, a camera. For a hidden leak, acoustic or thermal equipment and a meter test. For low pressure, a gauge reading. A plumber who names a large repair without measuring anything is guessing, and you are paying for the guess.
Most significant plumbing problems have more than one legitimate solution at different prices: repair or replace, spot repair or reroute, trenchless or open trench. A good plumber gives you both numbers and the trade-offs. One option presented as the only option deserves a second opinion.
Genuine emergencies are obvious: water is flowing where it should not. Everything else can wait a day for a second quote. Discounts that expire when the plumber leaves the driveway are a sales technique, not a plumbing consideration.
Anyone can collect five star ratings. Look for repeated themes in the text: did the price hold, did they arrive when promised, did the fix last, how did the company behave when something went wrong. Those patterns tell you far more than the average.
A real plumber, a flat-rate quote, any hour of the day.