I prefer the direct fit over the insert ring for three reasons...
1. Housing. Present standard storage systems for housing and transportation are more available.
2. Space. The direct fit capsules take up less room.
3. Economy. The standard size air-tites are cheaper to purchase.

As for the black (or white) foam insert rings on the oversize capsules ... according to the manufacturers and independent tests made on the insert ring in the coin industry, they are inert, which means they do not contribute to the discoloration of silver strikes. I've personally not read of any decomposition problems.