What is important in a gaiwan? I can imagine glass ones would be nice so people can see the leaves floating around but is that the only advantage a glass one would have over a porcelain one? Even so I think I'm a bit too old-fashioned to get a glass one. I've seen them with thicker & thinner walls, and also with lids that don't fit perfectly tight, so there's a narrow gap for the tea to pour through. I don't know that that is considered a flaw or what. I also like hand-sized ones that I can pour one-handed. & a patten on the inside is good too, so I know how much water to pour in. (that might be the only reason a decorative hand-painted gaiwan would be useful for making tea)
I have gotten some of my best gaiwans from Daniel. We've explored together - and I've examined on my own - the shapes of several antique pieces he has, benefits of thickness/type of porcelain, advantages of gap vs. no gap in the lid, etc and I think there are some very big differences. However, it also depends on how you like your tea brewed and what type of tea.
I mostly drink moderately-oxidized Taiwan oolongs and I've found that I like thin clay that is not very porous and has a tight-fitting lid. However, the antique gaiwan that are a bit misshapen/chipped and have porous but thicker walls tended to work quite nicely for the higher-fired teas (like a Wuyi tea). This type of gaiwan has, in my experience, been able to smooth out an overly-fired tea. The porosity and loose fit allows the brew to cool such that it can be brewed for a longer period of time without creating excessive astringency.
Glass is cool, especially as a visual aid for nice-looking teas. I think that, as you observed, there are many different types of glass gaiwan and the thicker ones may trap too much heat, which might not be too good for lightly-oxidized teas.