First, what make model and length of boat is each motor pushing? Also what 2 stroke Yammy does it have? Carbed, HPDI, etc? Most Yamaha 2 stokes are good runners. 150 HPDI Yammys may be one of there best HP in that series. They do require strict maintenance of those expensive fuel filters on the motor. Are there service records available for either motor? The 4 stroke 150 at 1200 hrs should have had a 1000hr maintainance done already including a timing belt. Either way you go, they should be checked out, or if being sold by a dealer, get a written warranty of some kind. DO NOT BUY A BOAT WITHOUT a SEATRIAL. If your spending alot of coin, think about a Survey.
Matt