The Phantasm has the homing abillity while the S.D.M.G has the super accuracy ability. The speed of the phantasm builds up as you hit enemies while the S.D.M.G has incredible critical strike and damage.

Is the Phantasm better than tsunami?

Tsunami is better for single shot damage with it's higher damage and shooting 5 accurate arrows. The Phantasm is better for general DPS esspecially against single high health targets because it gets the tracking phantom arrows when you hit one target multiple times.

Is SDMG better than vortex beater?

Compared to the Vortex Beater, the S.D.M.G. is more accurate. It does not fire a homing missile, however.