The advantages/disadvantages of a parallel-shaft vs. right-angle gear drive are more practical than technical.

The parallel-shaft drive is taller than the right-angle drive.  Therefore, the parallel-shaft drive will require more headroom above the mixed tank.  The right-angle drive, being shorter will fit with a lower ceiling or roof height.

The right-angle drive covers more of the top of the tank, surrounding the mixer.  It the tank had many pipes and sensors in multiple nozzles on the top of the tank, the more vertical parallel-shaft drive may be better.

On the technical side, the parallel-shaft drive may be less expensive for the same amount of torque because helical gears are typically less expensive than spiral-bevel gears (for a right-angle drive).  The output shaft speed should be possible with double reduction in either parallel-shaft or right-angle drives.  Both types of gears are efficient, so you have little advantage or disadvantage between those choices.  Right-angle, worm-gear drives are typically less efficient than the other two types of drives, but not with any problems for the output speed.

If you are dealing with high pressure in a closed tank, bearing design becomes more important, but most pressures can be handled with either drive type.

The right answer probably gets back to the question of space and access to the mixer, not the gear drive features.

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