The Beam turret actually does two loads of damage. The damage listed is for the main beam, and all the extra beams do the same damage again. So this does more damage than the sniper.
However, with the sniper having more range, it totally depends on the speed of the enemy and the placement of the turret which will do more damage in total.
A booster tower is MUCH better than a 9th turret if upgraded to max….
Hope that helps! 