I've also looked into both of these models as an upgrade to the stock pickups on a fretless bass I own. You're right: They both come highly recommended for fretless...
Although I can only share what I've distilled from reading numerous threads on the subject, I think you'd find that the NJ4SE imparts more of a smooth, refined, hi-fidelity tone to your instrument; while the Model J is a little "rougher around the edges" - in a very good way - yielding a powerful tone with lots of snarl and growl.
At length, I've decided to go with the Model
Js - because they are such a bargain compared with the Nordstrands, but also because having a fretless with a little more aggressive attitude would for me be a pretty good thing...
MM