Should I read Natsu no Zenjitsu or Mizu to Gin first? I usually prefer reading things in publication order, and if so it would be NnZ first and then MtG, but comments I've read online seem to imply it'd be more enjoyable to read Natsu no Zenjitsu first.

So... can someone give me their opinion, please?

I read Natsu no Zenjitsu first, story-wise its first and you'd get a better grasp of the characters (since they're in both, but different protags) with the thicker work. Mizu to Gin kinda functions like an epilogue since its so short too.