It really comes down to condition! One thing to consider - Mercury is still in business while OMC/Johnson isn't, but many will say that is not a big concern.
From personal experience the Johnyrudes are far easier and less expensive to maintain. Parts for them are also available still almost everywhere. Mercs, depending on model and year have many NLA parts and ignition parts are much more costly. Which is the way I'd be looking at it. Those enigines being of the same era, age would play no part in the decision. Price and condition would though.
Help me out...what's NLA?
Good point...who supplies parts for omc?
I am purchasing a 1977 lund S16. My 2 options for motors are a 1977 Johnson 25 or a 1983 mercury 25. Which would you choose and why? ..?