Bartolini has a low output and a more soft, mellow tone. The output always seems quiet and a preamp tends to be required to get any sort of tone shaping. Personally, I don't love the tone, but 1000 people on here will disagree with me.
Nordstrand is the new kid on the block and while I've personally never heard their stuff, I like what I see when it comes to specs. I think they tend to do a good mix of modern tonality and can be shaped with a pre to make more of a vintage sound as needed.
One thing to remember is that the preamp is not a magical device for making things sound exactly like you want. You should get PICKUPS that sound like what you want them to BEFORE adding a preamp.
My best personal example of this is my Squier Deluxe Active Jazz. I replaced the stock preamp with an Aguilar OBP-3, expecting the world. It turned out good... better than it was, but it wasn't like a completely different bass.
When I replaced the pickups with Sadowsky pickups... IT CAME ALIVE! They had a different voice and higher output which gave the preamp more to work with.
So choose pickups first, then do preamp.
(My pickup vote goes to Nordstrand, by the way. Because I want you to tell me how they sound before I buy some

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