Dust Cap Types: Which to choose?

The porous ones it is then!
What about size?
I've both large and small... can't measure them right this moment, but the small is just larger than the VC opening... (2 and something inches)
The large, just under 6 inches...
I've no idea which part of the cone is best served by having a cap in contact...
 
I have a question regarding the WOOFER dust caps... the ones I have in my Gradient Revolutions, which I am currently re-foaming, have vented dust caps. I am going to replace them with the same, but it gave me pause to wonder why use a vented dust cap. I figure it's got something to do with air pressure and the relieving thereof within the motor of the speaker.... but perhaps there is no real difference if using solid vs vented?

Hoping for a guru type answer. Electronics is my specialty , not speakers.....