Which is the more environmentally friendly (and effective) loft insulation: glass fibre or rockwool?

I can’t afford my first choice loft insulation (sheep’s wool), so I need to choose between glass fibre insulation or rockwool. Can anyone recommend one over the other? Is there a better third alternative? I know either of these can cause an allergic skin/throat reaction, but do they pollute the wider environment as well? I am extremely keen to minimise environmental damage.