Somehwere on Vimeo (and on a web page) is an extensive... extennnnnssssivvvve... test of the common cinema filters, which include the promists and so on. You'd have to google around to find it, but very interesting.
You really might consider or visualize what sort of look you're after - the black nets soften without much halation, as do any "black" filters; normal promists and so on add some haze. Then there's the warm promists, and the hasselblad resin filters, and the cokins... there's more diffusion out there than you could test in a year.
Interesting fact - the first Iron Man movie was shot almost entirely with a 1/4 black Pro Mist, but you don't think of it as a "diffused" "Remains of the day" look...