What year class? I am not fond of any Mercury motors before the late 80's. Old OMC motors were far more reliable, in my opinion. But in the 90's, Mercury really improved things.
For instance, the Merc 25 is a great motor. Tough and reliable and strong when maintained right.
In the late 90's, the 90 HP and the 75 hp are the same block, both with 3 cylinders and 3 carbs. Both are very good motors. The 115 of the same year class is a solid motor, 4 cylinders, but a little lackluster in performance. Not a lot of performance gain over the 90, at least not in the boats we put them on.
Later, in the 2000's, Mercury motors are better and better. Their 4-strokes are sweet and smooth, as long as you don't let them overheat or let ethanol water sit in them.
OMC motors have been really solid for many, many years. That's why you see so many from the 70's and 80's on the lakes regularly.
In the 2000's, OMC moved to the Ficht system, which was their downfall, as they released the motors in mass before fully testing. Lots of failures when they leaned out the 150/175 hp engines to meet upcoming EPA guidelines. But OMC was a stand-up company, and they FIXED every one of those motors. This lowered their stock values, and they were bought out by Bombardier.
The technology was good, once they got it dialed in. They renamed them Etec and they are good motors, IF maintained right. There is a huge controversy about them, with a LOT of haters, but if you got a good one, it was the best motor you ever had. But the stigma never went away, and Bombardier closed the doors on Johnson and Evinrude a few years ago.
I still run a lot of Johnson and Evinrude engines.
My 22 Offshore has a Ram 150 that is flawless.
My 20 pontoon has a 70 Johnson that is flawless
My 19 pontoon has a 40 Etec that is flawless
My 16 Princecraft has a 50 Johnson that is flawless, and
My current project, a Spectrum 16 Sport just got a 60 Johnson that is flawless... SO FAR.
The Mariner 16 had a flawless Merc 25
A pontoon I flipped had a great running Merc 35
Several other boats I flipped had Merc engines ranging from 5 to 200 HP and all ran great.
You have to mention the Yamahas and Hondas. They are arguably some of the best engines out there, with Suzuki and Tohatsu/Nissan getting up there too.
The reason I currently run all OMC motors is because they are easy to find, cheap and easy to work on, and very solid performers. For instance, I paid $700 for my 2001 Ram 150, $600 for the Etec, $300 for the Johnson 60, and so on. Most of them needed some cleanup and minor work, like a power pack or a coil, besides the typical water pump and carb cleaning.
A long post, but hopefully, it gives you some perspective. I'm sure others will disagree, but this is my take on them.
