Well, I have both motors, and I don't think either will change the value very much. It's just a matter of choosing which motor the new owner will want.
The last boat I restored was a 1990 Sailfish 170 CC. Nice, clean boat, rated for 90 HP. I had both a 70 and a 90 HP motor. Although the 70 HP seemed like a great fit, "Everyone" said to put on the max HP on, so I did:
When I took the buyer for his test ride, he felt that the 90 was crazy-fast. Truthfully, it was. It was so fast, that if you weren't careful with the throttle, you could easily throw everyone out of the back of the boat. It was like a hot-rod. A little bit scary. He wanted the boat, but only if I would be willing to swap the 90 out for the 70. I did, and he was very happy with the boat. But I ended up doing two engine installs.
I don't want to do two installs on this one. Just trying to pick the best motor to start with. My guess is that most people will "want" the 70, but that the most practical motor for this hull will be the 50. Same tough decision as I had with the Sailfish, unfortunately.