The issue comes when you finally go to sell a boat that is 20 years older than the motor. you'll get hurt then. heck you cant get a decent used rig for what they want for a motor.
I think you would lose money on either no matter what. Depends how deep your pockets are....Are new boats holding their value that well? One of my favorite boats I've ridden in was an 18ft pro craft with a new 175 merc. We could run over 70 completely loaded.