Why would you even compare these two shows?
It just depends what you're up for. If you're up for a good night out that you don't have to think too hard, some silly, escapist, entertainment, then go to Spamalot, because for those reasons, it is brilliant.
If you want to see something a little deeper, honest, and serious, then go see A Chorus Line, because for those reasons, it is also brilliant.
Why does everything with you people have to be about "better" or "worse"?
"I will join this conversation on the proviso that we stop bitching about people. Wigs, dresses, bust sizes, penises, nightclubs and bloody Kylie!" - Bernadette, Priscilla Queen of the Desert the Musical