Dirty is in underground hip hop, and usually from either using distortion as an effect, or the fact that it's really underground and they can't afford anything better than a radio shack mic going into a sampler, or whatever. Gritty, dirty, crackly, etc... I guess so it matches in with gritty beats and samples? Some people use that as their style, but it's uncommon except with a few underground groups.
I have used a shure sm58 with an art studio mp for hip hop vocals and i find it to be too muddy. Muddy mic, muddy pre, muddy recordings. But i record a lot of instruments as well. I don't really sample much, and when i do vocals in the future they will be more like singing, but writen sort of like a hip hop song.
Just get the best you can, do not go for gritty.But remember if your bad, the better your gear the worse you'll sound. You gotta practice like crazy. But recording yourself and hearing how bad you sound can help a lot to make you better. you can work on the things you hear wrong, and keep trying until they sound better, and a good mic would give you a better view of what's bad and good.