I don't care enough to draw a distinction between both of their newer albums, but I suppose Metallica's last album is better than anything either band has done in a long time. Although I'm actually having trouble remembering the name of that album, so take that how you will.
Oh, and I guess I'd have to go with Megadeth in general. 90s material wins easily, and I still care about more 80s Megadeth albums than I do 80s Metallica albums. Kill Em All rules above nearly all other metal albums afaic, and RtL isn't far behind it, but I've become bored with MoP, and AJfA just has too many problems to keep me coming back. But Killing Is My Business is some raw, brutal shit, Peace Sells is a hit factory, So Far, So Good, So What is like their second most pissed off record, and Rust in Piece just has some of the sexiest speed metal riffs of all-time.