Your tire clearance will likely be different for 650 and 700. For some frames there’s more room for 650, on some more room for 700. It depends on the radial clearance (top of the tire), stay shaping (sides of tire). So your max 650 tire may not have more volume than your max 700 tire. For the Aspero, both of my tire sizes are maxed out (while still keeping proper mud and rock clearance).
It also will affect geometry a bit. Gravel bikes usually have lower BB than road bikes, so 650 can exaggerate that, or drop a relatively high BB to the sweet spot. Handling is affected as well. So which wheel size is better for your preferences and bike geometry may be your deciding factor.