For me it is SM. LOTR is definitely a great pin but it quickly became a grind. The rules feel too linear after awhile. SM games can be long but even after a long game, I will hit the start button again. After a great game of LOTR, I just feel drained and it feels like a chore to have to work up to the point of destroying the ring again.
I've owned both. SM is staying. I'd still get a LOTR again, but not at the expense of SM.