You have to know what the computer is doing with the trigger signal to say anything about this. You could use the tach signal if the computer were time-averaging the tach signal over more than two pulses (ala Megasquirt). If the computer looks at the the time between pulses, that goes back and forth between two values, neither of which are correct for the speed of an even fire six cylinder engine. Certainly if your computer can be programmed to accept a certain number of VR triggers per rotation, there is advantage to more up to a point. On a purely signal-to-noise basis, averaging a noisy signal (your pulses) reduces the effect of the noise by the root of the sample increase. So comparing 3 pulses to 36, the noise will be reduced by 1 - sqrt(3/36) = 71%. This may or may not be significant, depending on the strength and accuracy of your VR pulse signal. At some point, I'd expect the notches are so close together that the VR pulses are degraded.