Having flown a lot on both types, it’s a bit more nuanced. Yes, the 787 has a lower max cabin alt but as the 777, especially the -300, often cruises significantly lower it can have a similar cabin alt for much of the journey. As TD says, it’s the same air coming in and being compressed so where’s the difference? No-one uses humidifiers
AFAIK as it’s a cost option, both initially and in running. Our 787s have the CAC output set to max, irrespective of the number of pax, as they’ve found they break much more often at reduced flow rates. A lot of people say they feel better on the 787 but is that because they’ve been told they should?
As far as hydration goes, the amount you lose in an ultra-low humidity environment is not that much more than normal but you feel much dryer. Desert climates can have very low humidity levels but unless you’re running around like a mad thing, you don’t actually need that much more fluid input. That said, if you feel thirsty you are in the first stages of dehydration, so little and often. We have had passengers with water poisoning after drinking too much having read on the Internet somewhere that you had to chuck down at least a litre an hour on a flight because of the dry air...