Wilson Watt Puppy, which version?

I am thinking of getting into the WIlson line with the Watt Puppy's. I have heard a few before in showrooms and I enjoyed them. I can not afford a new one, so looking at the previous ones there were several versions produced over the years. Can anyone explain the differences and if there are any "sweet spots" in the line up. ie is every newer version better or was there a point of diminishing return with the introduction of newer versions? I am trying to stay under 10k for a used pair. Thank you