Seems that everyong has an opinion about fasteners. Reading carefully, some of these opinions may be based on geography.

I'm in central Ontario. Humid summer, dry winter. Winter made even drier by my WOOD only heatsource.

On 3/4"solid walnut floors, plywood underlay over plywood sub. Which is the better choice? Staple? Cleats?

Am concerned with the diametrically opposite weather we get here, coupled with a dirt/gravel crawlspace, there may be extremes in expansion/contraction.

Cheers!

Im a staple guy nailing pattern more important for securing wood. Have a nice day wentzwood

we have high r/h summers followed by dry winters and if I were in your situation id by a quality engineered floor with a nice sandable 4mm top. Solid over crawl spaces will move quite a bit Ive gotten to the point where Im starting to discourage solid over crawl spaces if the customer is open to a quality eng. floor. Oh and I prefer cleats in plywood or solid 1x subfloors and staples in osb.