they're both very articulate under distortion- the emg does sound very scooped and in a band situation it's recognisable by a charactistic 'ssssssh' above other guitars - they seem to peak out at a different (higher) freqency to passives...
as for the 500XL... man. got my own from WFD now, rips the 81 a new ass. so much chunk, and a screaming solo tone
i liked the 81 for some things, but i LOVE the 500XL now