It almost seems like pot luck when it comes to failure or not. Some have failed at 85K some are fine at 250K. Maybe maintenance around the height sensors would make sense, i.e. keeping them free of crud build up. Mine was garaged for over 10 years so maybe that helps?
I still say the LS400 with springs rides every bit as well as air. I've had two LS430's and three LS400's and if anything the latter may just be best, but only just. The problem with air (if it goes wrong) is not just the struts at £850 each but four sensors at around £350 each. Then you've got valves and a compressor. They wouldn't all go at once but the combined total for replacement would necessitate a very close relationship with your bank! For me it would be springs if a strut went, that way all the above would no longer be a problem, plus replacement appears to be fairly straightforward.