Bellows and diaphragms are often used for the same purpose. Following is an explanation of guide in selecting them:

Shaft sealing	Some lateral room for absorbing the gap of the axis can be obtained. The diameter can be relatively small.	The axis can be fixed at a certain position.
Pressurization sealing & vacuum sealing	As dead space makes replacement unfavorable, contamination should be considered.	Easy replacement and cleaning makes contamination of fluid unlikely.
Pressurized function	Force proportional to pressure can be obtained because the pressurized area is almost steady in displacement.	Pressurized area changes depending on the shape, but is stable at the fixed position.
Resistance to pressure	Multiplying makes great resistance to pressure possible.	Thicker sheets make great resistance to pressure possible, but the response is reduced.
Isolating function	As volumetric changes can be large, external force is transmitted to the element easily.This function is not be affected by the junction compared with that of diaphragms.	The structure can be simple and the resistance to pressure can be increased by means of support.
Displacement function	Large displacement is possible.	Only small displacement is possible.(The displacement can be large by capsule combination.)
Linearity function	Controlling non-linearity is difficult.	Stable non-linearity is producible below 1%
Non-linearity movement function	Uncontrollable	Movement changes caused by pressure or force can be adjusted at a certain position.
Rotary feeds	Possible	Impossible
Torque transmission	Angle of twist is small. Anglar displacement and axis gap adjustment are possible. They can be used for precision control.	There are structural limitations, and few usage examples.
Volumetric change function	Because the volumetric change is large, many uses that require different volumes are possible: pumps,accumulators,gas cushions,dampers,etc.	In the case of a small volumetric change, diaphragms can be installed compactly and costs are low.
Vibration-isolating function	Vibration-isolating is very effective because it is flexible and its displacement can be large.	Vibration-isolation is small but effective.
Spring function	When displacement occurs,spring works on keeping the load uniform to retain the sealing.	When a certain degree of external force pressurizes diaphragm, it will pop.
Piping function	Wide range of uses that take advantage of adjustability.	Cannot be expected.
Thrust occurrence function	Axially pressurized area(thrust)of bellows is larger than that of diaphragms if their diameters are the same length.	Thrust is small but the equipment can be light and small.
Mass production	Formed bellows can be produced in large quantities. Rolled-formed bellows and welded bellows are not suitable for mass production.	Diaphragms can be produced in large quantities around several hundred thousand.
Price	The prices differ with their kinds and quantities. Generally bellows are more expensive than the diaphragms.	An initial metal mold has to be produced because diaphragms are mainly custom-made,but the diaphragms themselves are producible at low cost.
Manufacturing period of time	The periods differ with their kinds. Generally bellows need longer periods than diaphragms to be manufactured.	Diaphragms are producible in a short period except for manufacturing the initial metal molds.