Well the 82' Johny was rated at the flywheel so you are getting about 30hp from it and not the 35 that the badge says. The Mariner was rated at the prop in 88' so you are getting the 25hp out of it. That being said, the Johnson will most likely outlast that Mariner in the long run if both motors are in about the same condition and you will be getting a few more HP from the Johnson.
The 25 mariner had a 24.4 cubic inch block in 88' and the Johny 35 had a 31.8 cubic inch block in 82'. Both motors weigh within about 15lbs of each other depending on the model so there is really no big advantage in weight by going with the 25mariner.
If you are comfortable with the mariner mechanically speaking then you may just want to stick with the mariner. But just because the Mariner looks more refined does not necessarily mean it will be easier to maintain and work on. The 80's Johnson's are very simple motors to work on and will last forever if maintain correctly.
I would go with the Johnson, more HP, bigger block (which should outlast the mariner), and easier to work on IMO. But I have a jaded opinion obviously :lol:
Are they both the same style? Meaning electric start or rope start, long shaft or short shaft. Is either one a commercial model?