First and foremost, space. If space is an issue and you need a large volume mixed yet are squeezed for floorspace, vertical ribbon mixers bar far are more advantageous.
Second, is cleaning. While both have the ability to have spray balls added for CIP, obviously the vertical ribbon mixer will clean faster just because of its shape and using gravity to drain.
Third, we have to take a look at versatility in batch sizes. A horizontal ribbon mixer must be filled to at least 50% to be effective, yet 80% is really ideal to get an efficient mix. Vertical ribbon mixers only need a 20% capacity to achieve an effective mix.
Both units can have several options added such as ASME jackets to allow for heating & cooling. Both units can have choppers added to assist in mixing, declumping, and resizing. Both units can have CIP ready and both units are offered in small sizes as well as large capacities.