What's your opinion as to which is better and why?
They have a lot in common, both Stern Steve Ritchie DMD games with three flippers and three ramps, both have dots with available Color DMD (I think ST has better custom dots as opposed to muddy SM animations with a lot of screen rips from the movies). Both are movie tie-ins with movie call outs and both have actor custom callous, both mod up well, both have strong themes. SM has by far the better custom callouts by JK Simmons.
I love both and think SM flows a bit better (the Venom ramp is my favorite shot on either game). SM seems to be more same game every time sort of thing while ST we can pick our own adventure with different goals and shot multipliers. I didn't find that either of the early mini-wiz modes were that great but there is a sense of accomplishment getting to these early mini-wiz modes. Both can play long, but SM seems to be the longer one (almost too long).
If I have a larger collection, I would like to have both but to pick a better one it's ST mostly because of the way you can pick the missions.