Glass every time for me in all formats. And I use glass on the bottom even with 35mm. I don't use anti-newton ring glass in my carriers. I don't have problems with dust.
Just regular, thin-ish glass? Or something like 1/8 window glass? I considered rigging up something with window glass but I have seen Newtons Rings before, although not in this context. So I went with the carrier. (Dammit, I'm worth it )
I photograph things to see what things look like photographed.
- Garry Winogrand