i've always used amp gain for distortion, none of the pedals i've tired have beaten it, tired the big muff (black russian one), boss metal zone, boss ds-2, almost came close to replicating the sound i was using on my amp with a home made thing but it was prone to picking up noise and caught fire once as well.... only one i use regularly is a modified big muff for making doooom noises
i prefer using amp gain for simplicity and i just prefer the amp tone as i think it sounds a bit fuller than a wee buzzy pedal, been using a tech 21 tradmark 60, a laney gh100l and a homemade low watt valve job (although that runs clean).