I apparently never voted in this particular iteration of the WildRanger poll series.
My unpopular opinion is that MoP is not among Metallica's strongest releases. It's probably their least melodic entry (and melody is very important to me), maybe with the exception of KEA. I would rank it behind RTL, AJFA, TBA, Load and DM.
Between these two artists, I like Metallica a whole lot better. Better vocals, better lyrics, and, when they're both at the top of their game, better melodies.
But this is Megadeth's best release by far, up against one of Metallica's weaker efforts. So I actually will take Rust in Peace.