Not all digital pianos can drive high impedance headphones loud enough. My Casio Privia PX-S1000, for example, cannot drive the AKG K-712 Pro loud enough. For this reason, I am also looking for an open headphone that can go a little louder than the K-712 Pro.

If, then 80 Ohm. Better treble reproduction. But I wouldn't take either of them because they are both quite bass-heavy, which I don't find so pleasant on an electric piano. I think the sound of the Sony MDR-7506 is great, and it's not expensive either.
Sony MDR-7506 (Over-Ear)

If it costs a bit more: Sennheiser HD600. Top sound quality. But not closed.
Sennheiser HD 600 (Over-Ear, Black)

If you want to go by theory, then go for the 80 ohms. However, in my experience, the ohm ratings of headphones are similar to the wattage ratings of speakers: it depends on the design how good and loud something sounds.