I’ve also heard saltwater is easier than fresh and it isn’t.
I don't think anyone has ever made this claim without having an ulterior motive (a LFS owner wanting to sell you an expensive saltwater setup, for instance). There's no way that saltwater is easier than fresh. Is it that much more difficult? No, absolutely not. I'd say they're probably pretty close in effort, with salt just being more expensive and needing more tools and time. But there's no way that salt is easier.
What I want to know is this some saying a businessman started to sell more corals at their fishtank store? and after that the placebo effect has set in to make it true.
This is a fair point, especially about the placebo effect. People are horrible at discerning cause and effect, as well as making rational and objective decisions about outcomes (this is, essentially, why the scientific method evolved). Having said that, have you ever maintained a large tank? Have you ever siphoned a larger sand bed? Have you ever wiped down the glass of a larger tank? How do you know you don't just believe your smaller tank to be easier because you've never cared for a larger tank?
Personally, I would always take as large a tank as I could get (under 10,000 gallons or so). I work in the Living Seashore exhibit at the National Aquarium in Baltimore. The smallest tank we have in my exhibit is 93 gallons. The main touchpool is 2,300 gallons and the Community Living tank is 1,800 gallons. These tanks are much easier to take care of than my home tank. They take more time, but it's not a lot amount more. Take the Community Living tank. The siphon is over 4 feet tall for this tank. I need to get a ladder out of storage to even get my hands into this tank. Is the sand easier to siphon on my 20g long? Of course (or, it would be if I had sand). Is the rock easier to scrub in my 20g than it is in the 1,800 community tank? You betcha. Is it easier to just use my magnetic scraper at home as opposed to using the telescopic pole on the floor mop to clean the inside of the Community tank acrylic? Of course.
Is it that significant of a difference? No. Even getting the ladder out, the tank is cleaned in less than 20 minutes. Sure, it might take you 5 minutes or less to do this in your home tank, but this isn't a huge difference considering the huge increase in tank size. Plus, not every large tank is 1,800 gallons either. Siphoning just doesn't take that much time. A 120g tank has less than double the floor space of a 40g breeder. So if you spend 5 minutes siphoning a 40g breeder, a 120g would only cost you 5 more minutes of maintenance in that area.
Are there reasons to have a smaller tank? Absolutely. The costs in this hobby just balloon as the tank gets larger. I personally wouldn't do more than a 40g breeder without some serious financial planning. But, that's a different discussion for a different time. Having worked with extremely large tanks, I would personally take a large tank any day over a smaller one.