I don't know all the rules for FV in Solo. But, there will be no "better". The smaller engine with the duals is going to produce it's peak HP in the ~5500+ RPM area. The 1951 will do it in the ~5100 area. They will have different curves, with the 1915 probably having more torque and as much HP "under the curve". This assumes the same cam and timing. So the "better" engine will depend upon the track design, your final drive gearing and where you can shift at.Speaking about the engine…which would be better for autocross?
Since BTU released will be about the same in either engine, cooling issues will be similar. We have built many 150 to 180 HP ACVW's but none in the exact configurations you have here, so I don't have the dyno nums to be precise.