High density has advantages of scale, i.e efficiency. On the other hand, when things break, they break massively I.e. a power outage, a garbage strike, etc. Also, infectious diseases propagate faster.

Still, I would take density over sprawl, if push came to shove. I wish San Fracisco would adopt some of the Chinese practicality when it comes to urbanism --not the ugliness of the uniform apartment block buildings, but the allowing of change and growth.