This one isn't hard for me. Purple Rain is a great concert film featuring an all-time great plus Morris Day and Jerome to keep the plot from getting in the way too much plus Apollonia to look at as an extra bonus. What's not to like?
The Wiz is, well, The Wiz. That is a kind of odd adaptation of a Broadway show that is not without its charms but still feels like a very traditional musical to me. The biggest problem is that I don't find the music very memorable outside of that Stephanie Mills recording of Home with Take 6 doing the backing vocals. (I remember Ease On Down The Road but I don't like it.)
So, yeah, I'll go with Purple Rain.