Personal opinion: I really like the natural looks of bottom substrate, and goldfish love digging around in it for food, so I just feel like it would create a nicer and less sterile environment for your fish to have some. But of course, if you want to have your tank without any grovel, feel free to

But I think you could combine these things! Add some stones and some plants : D Besides, driftwood looks nice. And plants give some cover for your fish in case they get nervous! Airstones are quite important too since goldfish need a lot of oxygen for everything. So don't hesitate to get one! (: