Chevy v Ford anyone? Anyone that is decisive about one being better than the other gets the skeptical side eye from me.
They both are excellent, and really have great track records in the 2000’s. Your issue above might simply be due to motor shortages and availability, or dealerships switching to Merc’s in your area for whatever reason. It seems to me that motor brand prevalence seems somewhat regional between Yamaha, Mercury and Suzuki in particular.
Personally we have a Mercury Verado Pro. Couldn’t be more pleased with it. If I had a Yamaha SHO instead, I am sure I’d be equally thrilled with it. We went Mercury mainly because our local Bennington dealership is Mercury certified, and sells primarily Mercury motors. Since I am more used to seeing Merc’s, it also checked my motor comfort box too. AND…since they’ll be the one’s servicing my boat, I stuck with a Mercury on the back of my boat.
Had they been a Yamaha dealership, I would have went Yamaha. If the dealership nearest you is Mercury certified, then I’d go for it without reservation. That said, I cannot speak to salt water experience or use. We are from Michigan, and our waters are Unsalted.
Besides, black cowl Mercury’s simply look

. I say that with complete personal bias!