Is there a real difference between a NGC and PCGS holder?

I buying a lot of my coins using Teletrade Auction and I only buy NGC or PCGS certified coins. (I know for some this may be a bad practice, but so far I have been very satisfied with my results.) Anyway, I have noticed that a particular coin of the same grade will generally sell for about twice as much if it is in a PCGS holder than a NGC holder.

Can anyone out there give me a good reason why PCGS should command such a premium? I thought these two fine companies used relatively the same high standards.