Wis,
Better weigh each of those heads, Muzzy vs. the Sonic. If they are truly 100grs each youshould not see any speed difference. 100grs is 100grs at chrono distance....maybe downrange if one is imparting more drag but not at a couple feet.
To see a 3-4fps difference just by changing the heads out I would guess that you'd need to see about 10-12gr variance in head weight. Sounds to me like one of the 2 isn't what you think it is. (Either the Muzzy is heavier than 100grs or the Sonic is lighter or a combo of both)

It certainly does sound your set-up likes them though and good luck with them this season.