I have not seen either although I will probably see both.
Nonetheless if I were in your dilemma, here is how my logic would go--
One is a limited run of a musical by a very talented young composer with a book by a playwright who is always at least interesting;
The other is a stage version of a movie that wasn't any good in the first place and features a flying car.
To me its a no-brainer.
At the very possible worst Piazza seems like an 'interesting failure'. At best The Flying Car seems like a flying car.
"If my life weren't funny, it would just be true. And that would be unacceptable."
--Carrie Fisher