I have 2 fat Strats, one with Crawler and one with Nailbomb, so I'm pretty well placed to answer this. The Crawler is more balanced with a more open crunchy rhythm tone, steroidal PAF suits it well as a description. Great drive and cleans with lots of versatility.
The Nailbomb, is like a further steroided PAF, with darker crunchyer drive, and less defined cleans. Great for the less clinical side of hard rock+. The Crawler is definately a more rounded pup, and dirts up well enough that i would almost always recommend it over the Nailbomb if you're not sure... But the Nailbomb has an ace up it's sleeve in the form of it's lead tone.
There should be epic poems written about the Nailbombs lead tone, cut, balls, drive, harmonics, scream. it does it all better than any pickup i have ever used. It's sooooooo good it's just untrue.
So basically if it's lead you want to focus on, then Nailbomb, for everything else the Crawler has the most rounded tone of any pickup BKP. It's the only pickup that can sound like a Nailbomb and a Mule at the turn of a dial.
