I was listening to Def Leppard long before I was into AC/DC only because my dad preferred them but I've got to say that Def Leppard peaked with Hysteria in 87. Adrenalize wasn't a bad album either but after that they just started looking stupid, like a bunch of old men trying too hard to be hip and trendy.
Black Ice was an awesome album, in my opinion possibly one of their best and shows that AC/DC have far better longevity than Def Leppard or most other rock bands for that matter.
Either way, I still like both in their own way.