Two bands I've not yet had the privelege to experience live yet, but are both two of my favourite all time bands.

I haven't heard much live tracks either. Joe Elliot has sounded awful from the few live tracks I've heard and Vince Neil hasn't sounded so pleasent either (and I'm one of the very few who enjoys his recorded vocals).

Joe Elliot doesn't forget words, Vince Neil does.
Def Leppard don't have TommyCam, Motley Crue do.

It's a very close call

I'm hoping to see The Crue on their 2009 tour anyway, then next time Def Leppard tours and I see them I'll come back to this thread and let you know.