Good point. Also one should consider the individual. People have slightly different ear muscle/cartlidge density and this can determine which is better Acrylic or Silicone. Most people have a soft ear muscle/cartlidge and Acrylic is a better choice for them as the hard Acrylic suits the softer muscle/cartlidg. On the flip side a few people have a harder muscle/cartlidge and thus with Silicone is a better choice being softer than Acrylic. This is one reason why some people prefer Acrylic to Silicone and vice versa.
The debate here though is not on which is better, it is on isolation. Personally I would say Silicone can provide a slightly better isolation although IMHO the difference is negligible, I personally would always take Acrylic. Isolation is 90% of the time down to the level of fit, this is why it is always important to have
open mouth impressions and DO NOT talk whilst having them taken.
I wrote a post some time ago on the importance of a fit for a CIEM, it could be useful for those that have not seen it to have a look at it.
post #3330