I've only ever used a dremel, but I've done loads of blowholes and I've got it down to about a half hour now to cut and clean up - the first one though will probably take a while and might not look great. If you're gonna use a dremel just remember that the lack of an guide means your holes probably won't be perfectly round to start off with (though with practice you can get them looking pretty smart), so you might want to practice on some spare metal, or an old case or something. And if looks a bit rough just add some blowhole trim.
When using a dremel, use any sort of cutoff wheels (normal, heavy duty, or reinforced), and take it slow - if you try and cut too fast you'll see sparks flying and you'll burn through discs in no time. You should be able to cut a 120mm hole with one (maybe two) discs - any more and you ought to cut slower. Then use any type of grinding/sanding bit to smooth off the cuts. Have fun modding
