I have ported quite a few of both of those engines and it seems the ninja motors consistantly lasted over 10 gallons without any trouble. The guys that ran the V-spec were reporting that the motors were fantastic for the first 3 gallons or so and then they started spitting out bearings. That was a common problem with those motors and OS has told me that they have resolved the problem recently with better bearings and I'm am still waiting to see what type of life we will get out of those. So in my opinion, the ninja which is Rossi based is the more reliable motor at this point and with the proper porting, they run pretty close to each other.
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