So here's the question : Is there a big difference between glass and metal slide?
I mean can't someone just play for example Seven Nation Army with a Glass slide?

the only differec is the tone it produces and that of course a glass slide breaks alot easier

i personally prefer a ceramic slide but i have a metal one as a back up in case i drop pr break mine
a glass slide will sound smoother, a metal one will be more, well, metallic sounding. it's all down to preference, personally i go for a metal one
glass slide has bit warmer sound, metal slide sound is more metallic, ringing.
there's also a difference in feel and weight.

You can of course play any slide song w/ any slide. It's just matter of personal preference (a lot like strings or picks)
I have a ceramic dunlop mudslide which i prefer to my old glass slide mainly because it fit better and the glass slide was too light. But the tone is different between the two.
"I am hitting my head against the walls, but the walls are giving way." - Gustav Mahler.

Quote by AeolianWolf
absolutely what will said

Yay, my first compliment!
moonshine slide all the way white clay with a cobalt blue glaze
stickin my nuts between my strings and strumming myself to extacy one note at at time since. . . .
different slides can feel different, along with sounding different.

I find that glass slides feel smoother and somehow softer, while ceramic slides feel hard to me. don't really like them as a result.

if you can, try popping for a metal slide and a glass one. they're different enough that you can really get a sense of which you prefer. from there, you can either experiment with slides made of different metals (steel vs. brass) or different glass and ceramic slides.
They all sound and feel pretty different. Personally, I usually prefer metal slides, but you just haven't lived until you've played with an empty vodka bottle.