Just my two cents from the research I've done:
Other than a short run in the mid 90's, Jason only used Spector's for recording, and they were his short lived signature model (I think they only made 15 or 20 of them, but my memory is fuzzy).
I have a couple late 80's Kramer made NS2's - the original HAZ preamp growls like crazy, especially if you run it with the bass and treble maxed like I did. The Sadowsky preamp to me is a little cleaner sounding, but has it's own tone that it provides that really makes them sit well in the mix. I like a lower action and use a little fret buzz to add some growl/grind on both basses. The EMG's in the Spectors are a little cleaner and more hifi sounding, while the Sadowsky pickups are warmer and have a great midrange.
The new MJ5 I ran over the weekend for a gig sat beautifully in the mix with my band, and just felt great for a 3 hour gig. I've used my recently acquired Metroline JJ quite a bit as well.
I love my Spectors... but they are heavy (both of my old ones are 10lbs) and the first fret can be a bit of a stretch at times. Nothing sounds like one though. BUT, I'm starting to become a Sadowsky guy live because they are so easy to play for long periods while still sounding great on their own.