Warp resistance of plywood vs solid SPF lumber
Plywood is inherently more stable than solid wood due to having been constructed in layers with grain at alternating right angles. Specifically, plywood with many thin layers will be more stable, and hardwoods will typically move less than softwoods.

I'd consider BC grade (commonly found at lumber suppliers) to be a minimum standard of quality for this purpose. You could look to a specialty supplier for thin-veneer hardwood plywood for maximum stability.