GH20LS10 vs. GH20LS15 vs. GH20LS30

Hi there,
gonna get a new DVD burner und well it turns out to be a LG GH20LS"XX" or Samsung 203N.

What I’m not quiet sure about is, what does the numbers at the end of a LG model tell?

Which one (number) should I buy or avoid?

Thanks a lot