I have to go with LOTR in this one. Despite some of the negatives (mostly the long ball times), the positives far outweigh them.
LOTR has some of the best call outs in pinball of all time. It has one of the best theme integrations of all time. It has at least two cool toys (the Balrog that swings out, and the Ring magnet). And it has one of the best rulesets of all time (which compliments the long ball times). The rules are so good, that you can have fun and feel like you accomplished something even if you don't have a long game.
I think the LOTR theme fits pinball extremely well. And while I do like Metallica, I still prefer LOTR by a wide margin in terms of theme.
I've owned both (I've owned LOTR multiple times) and I have better memories of playing LOTR and getting to Valinor than I do of MET.