650 DP or 750 Vac secondaries? Which one?

750 would be my choice.

Yes, the performance would be similar. The difference , however super mall would be;

smaller carb, a bit better throttle response and torque would be better down low (But not where your making any power but the put put around town type)

More noticeable difference would be the 750 will have greater mid-range torque and high end HP, IMO, somewhere round 10 HP and lbs.