The S is the superior luxury vehicle... but also much more expensive for suspension, electronic issues and general quality issues with the plastics (anything not wood, leather or metal unfortunately). The S Class engine reliability varies GREATLY across the range but you don't seem to be looking at the problematic ones.
For my money it would be the 7 - also one of the prettier generations of 7er.
My default advice is to thoroughly check out the air-conditioning system for whatever car you are looking at - this applies to any modern car. The cost is one thing... but ripping apart most of the interior/dash to get to the parts can leave you chasing down squeaks or rattles caused by internally broken parts just cobbled together while doing this (it is not a fun job for anyone involved as I am sure the dealers can only book a fraction of the time this takes... the techs probably also need 6 weeks of physio after this).
Let Uncle chip in here. Been here done that.
Got a W221 S350 Bluetech in poverty AMG styling pack. One of the struts checked out about 8 months into my ownership, it was 11k to replace and 35min at a shop in Kempton. While it was in Capetown the workshop that did the pre-delivery service fucked up some major things like shoving a D1S blub into a D2S socket, claiming to have done the cabin filter changes and such and one of the things they screwed up was the charge pipe so it was over the course of 4000km spraying some oil onto the alternator, so that checked out a a week before i was to fly to Zilleville and pick it up, 15K because while a common engine, it is S class only bits and pieces. BMW thankfully don't do that. I believe everything that hangs off that engine is common with all other similar aged G series cars with the same drive train.
It has some strange issue where the rear headrests and drivers lumbar support will sometimes work and sometimes not. I suspect this relates to some Capetown "i know a guy that can do it cheaper" fuckery that was going on since there is a bit of a rattle from the rear parcel shelf and alot of the electronics do live there. Not really bothered much with that in 2 years. Other issues i have had with the car was a result of previous owner and a dubious tune/tuner.
But now to the question at hand. Not once in the buying process did the thoughts of "cost of ownership " cross my mind. I think back to when i was 15 sitting in the back of a friends dads W140 S500 and i asked him about fuel use, "one does not drive a S class and talk about such things as fuel economy". The same rule will apply to a 7 series. No matter how reliable it will be, it remains a old BMW with old BMW issues, and expensive when new old BMW issues. So you have to understand a set of good rubber will be 20-40k, suspension bits are going to be 10-40k, the odd little annoying failure like a softclose door is going to be a complete pain in the arse. The most minor of accidents that takes out a headlight might write the car off kind of costs are involved here. It is going to be expensive no matter how reliable it is.
Me i voted with my money and it is not a 7 series sitting around, playing who can bankrupt me with the Z at home. It's a big blacked out S for a reason. It is simply "sonderklasse"
