Personally, I would not go over $100 US for an NL7037 or a Z568m. Why ? Because you can get a brand new RZ568m for $145 US, and it has a 10 year warranty. Unless the seller purchased these as NOS, you really dont have a verified history trail. Now, if you have an existing set, and need a replacement to match the appearance of the others, then paying a bit more makes sense. But if this is your first purchase, I'd stick with the RZ568m.
I recently purchased my first 2 RZ568m's from Dalibor, and eagerly await their arrival (they just cleared US customs yesterday), and after I've fired one up I will post a short review on neonixie. Based on what I've seen in the video, I honestly believe there are manufacturing techniques that Dalibor uses that make his tube superior to the NOS devices, and I'll put that in the review.