Between these two pedals, it would be a Finally for me. I borrowed a Nobelium from a buddy, which isn't *exactly* the same, but is similar enough to compare (I also have an Underground Accelerator, which the Finally is derived from), since the core of the circuits are effectively the same.
For what I want out of a preamp-style pedal, the Neptunium/Nobelium are just a bit too transparent. I like clean tone that is approaching the edge of breakup if I really start laying into the strings, but stays clean and thick when I play with more control. Plus, I also tend to have EQ earlier in the chain, and don't like stacking 2 or more always-on EQs onto one another, so the flat frequency, full character nature of the Finally is what would do it for me. I also seem to find that 5751 tubes hit the sweet spot in terms of gain factor in Sushi preamps; they still allow for actual breakup, but offer a much wider "on the edge" sweet spot than a 12AX7.
That all being said, many folks seem to prefer more crystalline clean tones than I do, so in that sense, the Neptunium probably makes a lot of sense.