TIS, I think that's because the Zefirelli version is a much purer (and safer) adaptation. That's why I prefer Luhrmann's, since that differed from the conventions of theatre (while also owing much to it, since the director's origins are on stage not celluloid). The 1996 feature had all the confrontational, risky ambition that the 1968 version lacked. Both are stylish for their times, but I think, at least on a visual level if not an audially poetic one, the 1996 film will last longer.

Mick