I am going to try to make a sempervivum sphere like How To Make a Succulent Kissing Ball | Apartment Therapy

the ones on this site^.

I will use two wire hanging baskets wired together, and plan on growing the sempervivums into sphagnum moss.

Obviously, if one used potting mix inside it'd get very heavy, so my plan is to use a spherical core of foam. You can buy 'wet' foam (oasis) or dry (presumably, polystrene). They cost about the same.

My question is, which should I get do you think? The wet will obviously provide more moisture for the plants- but could that be too much? And would that make the whole arrangement very heavy once water-logged?

Thanks for any advice!