Sch 80 is for industrial use ...sch 40 for residential use....both can fail for sure but IME sch 40 life expectancy measured in months and sch 80 in years in our hobby.
I have an inground swimming pool which needs nearly half of all sch 40 unions, valves, elbows, and bulkheads replaced every spring... my new 150g cube tank all plumbed in sch 80. Think it'll be several years before i see a single drip.
PS. Dont get a swimming pool its a hassle.
SPIERS brand fittings in both types are highest quality you can get.
Other words of advice if you have questionable threads use a dab of silicone not pipe dope with teflon. Thread sealant is meant for free flowing potable water not closed systems like our tanks. It will leetch into water as it never hardens. Teflon is not a trace element for your live stock and it is not even used in cookware anymore. Just cause it says you can use it on pvc doesn't mean you should. Manufacturers dont have our pets we love in mind. ... (mic drops)