cap varieties

[scopey at hooked.net]

I've been wondering this for a while, and no one seems to know:  Is 
there any difference between polyester and polycarbonate caps?  When it 
says 'poly', what am I to assume?  While we're on the subject, what's 
the advantage of poly-whatever anyway?  Temperature stability?  Improved 
voltage specs?  Also, is it generally OK to substitute a mylar cap for a 
ceramic one?  Again, what is the advantage of ceramic?

-Jeff