It could be various things causing the problem, unfortunately it's unlikely any of them will cost little to fix. I would say the first thing to have checked are the height sensors, just one failing can affect the whole car. It's also possible that a strut has failed, again, it affects the whole system. There are also various valves that control the system, plus the pump. You really need to get it checked by a competent mechanic who's familiar with air suspension, and it needs to be raised on a lift.
And to answer another question, I've had three LS400's and two LS430's and I really don't think air makes any difference over springs, except for the cost of repairs!