I have both. Abd tried both in the same guitar, my ESP LTD EC-1000VHB
The Painkiller has massive responce, however sounds like a weak fart when the tone is clean. With distortion its a great pickup, however i found that it was lacking clarity at lower tunings. However for classic metal and thrash, its perfect, anything down to D Standard it is great at. Drop that low D to a C and it goes wrong.
The Nailbomb however is my current personal favrouite thru my mesa, warm with a good responce. The Nailbiomb has alot more smoother high end without a too bold a low end for me. It is stronger on cleans, however isnt quite what i want but it is much more useable over what i have tried and for this im happy. The Nailbomb is great in Standard down to C Standard.