Cellulose is both greener (it's carbon-negative, since it's "sequestered carbon"), and more air-retardent than rock wool. Specify "borate-only, sulfate-free" material. Sulfated fire retardents are corrosive to metals when wet, which becomes an issue if the roof ever leaks. When the blower is set up properly it will settle an inch or so over a decade or so, then stop, so the installed depth needs to be slightly more than the targeted final depth. Most cellulose installers in my area are scrupulous about that.
Rock wool is completely fire-proof (it won't melt even if the whole house burns down around it) and won't settle over time, but doesn't buffer any moisture, and is somewhat more prone to rodent nesting.
For even-money I'd personally opt for cellulose, but there's nothing wrong with blown rock wool.
If they're talking rock wool batts, rather than blown, the fit can never be as perfect as with blown goods, and performance does suffer.
More important than the insulation material, the quality of the air-sealing at the attic floor prior to installing the insulation is PRIMARY to getting the thermal performance needed, and avoiding wet insulation and moldy wood where the wintertime air is leaking out and condensing/adsorbing into the wood and insulation. The better contractors in my area use blower doors an IR cameras to chase down and seal every air leak (including weatherstripping attic hatches) before installing attic floor insulation.
