This is going to have lots of opinions! I'm looking forward to evaluating each and every one!
I have a stand, on casters, with a padded bar where I have 6 bikes hanging by the front wheel. The spacing is kind of tight and you run the risk of scratches, plus I have to move at most two bikes to get to the third.
I have another stand with padded cradles that I use for the two cruisers because they're the bikes I reach for to take a quick spin with the kids.
Hanging by a wheel has the advantage of bypassing frame geometry, so mixte frames, ladies frames, modern MTB frames will all fit fine. I have no experience with hydraulic systems, so I'm not sure if hanging vertically is acceptable. I also like that the tires won't get crushed as they lose air. The displays in bike shops are probably ideal, but maybe not for long term if you're just storing them.