I may be wrong but from what I've read crash glass does not have the film in middle. Others who it will chime in soon I expect.

Is TG (glass without the film that holds it together)preferred for mosaicing and why? Does it look different than tempered glass when it's adhered? I keep reading only to use side windows (TG) but it seems that the glass that holds together would work well. Why do people say to use tempered glass?

You're talking about the glass in the windshield that has the film. It's called laminated glass. You only want the window glass. It breaks in very small pieces, and isn't sharp. You can break it by hand after the whole window is broken. Go to your local auto repair, or glass shop that does auto repairs, or an auto salvage yard and get it for FREE. It's very addictive to mosaic with. You can put all kinds of things under it. Check out the following site. She's the Queen of TG mosaicing.
Mosaic artist

Thank you so much. Now I know that I need tempered glass, not laminated. I knew there was a difference, but thought maybe I could use either for maybe different applications. RATZ, but I'll go to a salvage yard and see what I can find!

Some glass shops will give it to you as well.
My mother replaced her patio door and they cut the wrong size and being the good ole girl she is, she told them to leave it. They just toss that stuff.