Generality: an annular booster provides a stronger signal and will deliver more accurate metering at low air speed, and a cleaner transition to main metering without excess richness. The penalty is that the thickness of the booster slightly reduces CFM. In a Dominator this matters less, of course. Holley boosters appear all the same size, which means the obstruction is more important in a 390 than an 850, etc.

BTW: for highest efficiency a booster is supposed to terminate about .030" after the "vena contracta" (smallest point) of the venturi it's in. The discharge tube should be positioned the same distance after the booster's minimum diameter.