Re: Johnson or Mercury
I think they knowingly put hp labels on those older mercs lower than actual hp for whatever reason, particularly in the 40's and 50's. I've never seen a merc that wasn't a more powerful engine than rated, other than my crappy '73 3-cylinder 65hp mercury. But unlike some of my mercs, my 65hp actually cranks up easily even when cold.
I also think that OMR may be slightly incorrect if he thinks there are more older mercs in RUNNING condition than OMC.
I suspect that to be because:
1. OMC's ignitions in the 50's, 60's, and 70's are all very, very easy to work on and cheap to fix, vs. the merc ignitions changed a lot and had expensive components like triggers and switch boxes (notorious for boogering up), etc., and were hard to diagnose. I've also had cdi boxes go out for no apparent reason.

(I know, all electrical parts can do that but mercury parts are high compared to the universal magneto by OMC.
2. OMC went to stainless drive shafts much sooner than merc.
3. The 2-piece lower unit housings were easier to get apart than the merc lower units without boogering them up.
4. Last of all, the merc cooling systems can be a real PITA to get just right, and if they aren't, will quickly ruin a cylinder or three.
I personally like all American made oldies with the exception of Chrysler, and love the power, sound, and looks of the Mercury motors. But, they aren't from MY experience as easy to find in a condition that they can be easily/cheaply fixed as an OMC. In my area, it would be hard to find a tower of power in good running condition, but then again, in my area it's hard to find any motor over 10yrs. old in decent condition because the boating season is year-round and motors get worn out relatively quickly.
I will quickly admit that I don't know much at all about those towers of power and that if I found one with good compression on all 6 cylinders and had a lot of extra cash, it would be fun to learn how they work and get one fixed and run it. I would probably check the prices and availability of those electrical parts and lower unit seals and such before I dropped any dough on one. I also would expect to have a boat with a HECK of a sturdy transom to strap it on, because those mercs make incredible torque and will really mess up a transom!
Anyway, that's my viewpoint on the merc vs. OMC thing.
JBJ