Well, I own both a Tron LE and MM, and if I had to sell one or the other, I don't know which one I'd sell. Probably Tron LE, because it took me so damn long to find my fully restored MM.
Right now, MM is one of the top pins and has been regarded as such for a LONG TIME, while the Tron LE is new and hasn't stood the test that MM has as being a big hit. If you want faster game play, and the latest and greatest thing to hit the market, Tron LE is your choice. If you want a timeless classic that will always be fun and popular, MM should be your choice.
Really your question is not worth asking a forum of people, as peoples tastes are so different, it's just like asking which pizza people think is best. It all comes down to which one YOU like the best. I've never based my pinball purchases on what others liked better or thought was better, as I'm going to be the one playing the game 99% of the time.
Attack from Mars is also a very fun game, I love mine! But I would place it third behind my MM and Tron LE personally. I know several guys that own AFM's, but only 2 that own MM's, they are also much harder to come by (and Tron LE only has 400 produced).
Good luck!