I gave up on plain glass
slides. Diamond slides (and Nunwells) are hand blown lead crystal. Very different glass- much heavier. The tone comes from the weight on the strings. thats why brass and chrome or gold plated brass sound so good. Density. Same with glass, but glass is smoother.
Its all taste, but there is physics involved too.
Ian from Diamond Sllides wrote a very interesting article about the different types of glass used fo slide.
It can be found here:
http://www.ukguitars.com/article/slide_guitar1.htm