Man that's a tough one. TMNT is a harder and more unique shooter, but very satisfying when you are on. Stranger Things has one of the single coolest shots in pinball...nothing else is quite as cool as a demogorgan mouth shot. The rest of the layout is very solid, but fairly standard.
The atmosphere is very different between them, but capture each theme well. Stranger Things captures the spooky, atmospheric side of the theme more-so than the light-hearted kids coming of age side. Some people love that, some do not. I love it...no other game captures that spooky feel in the same way. TMNT is way more light and cartoony. If you were a fan of the show and early movies back in the day like I was, it will bring a smile to your face.
As much as I love Dwight, I will be honest I like Stranger Things code better overall. It has tons and tons of modes, and the progression through the game is awesome. There is a mini-wizard mode "check point" for pretty much every skill level. It's a wonderful build as you get further and further in the game...better than any other I can think of. Even if I'm having a crap game, I can still usually get to Total Isolation 1 and feel like I accomplished something. And Final Showdown is the perfect difficulty. After owning it a year + I still haven't quite gotten there, but I've been VERY close. I know it's obtainable to me, but it will never be something I see a lot of. Every mode is interesting, thematic, and fun. Heck, there are FIVE modes based just around the Demogorgan, and every one feels unique and not repetitive. This is by far Lonnie's best code.
Like I said I love Dwight, but I think he went overboard on the hurry-ups with TMNT, and based too many of them on the upper flipper shots. The scoring is also very imbalanced versus the difficulty of pulling off some of the goals. The actual modes (episodes) are good, but need to have the scoring redone. I do not think there is any way to boost your scoring in them other than if double playfield is running when you finish it. It's always 500k or 1m pretty much. Blah. Foot 123 combo feels amazing, but comes up way too often. That upper loop is crazy hard to hit on demand, and not one but two of the major goals require hitting three of them in a row, and for max Krang combo hit a side ramp on top of it. The other you have to pull off in multiball...and not just any time but at the right time. Good luck. That's just too much. Final Battle is fairly easily obtainable, but Cowabunga...forget it for mortals. The goals are just too difficult, especially Max Krang and Triple Mondo. Just way too many hurry-ups requiring too many very difficult shots to complete. They are all good in theory, but in the real world with this unforgiving of a playfield, it takes some of the fun out of it. And the button smash mode didn't really need to show it's face again after Star Wars...
On the other hand there are some absolutely amazing things about the code. Great moments and light shows...definitely Dwight specialties. The wide variety of modes is awesome. Co-op, 3 v 1, Final Battle Challenge, Half Shell Challenge (!!!), etc...so many fun ways to play. Good variety in the episodes...very cool that a couple of them need to be "unlocked" by playing a prequel mode. The training is an awesome concept, but the later levels fall into the insane difficulty level trap, making them almost impossible for most to benefit from. I like the many paths you can take in the game, and the choices throughout (which turtle, what toppings for pizza multiball, which episodes to bring into team-up, etc). Good mini-wizard in Team-Up (though I am terrible at it), and Final Battle with the multiple stages is also great (but fix the damn voices already...been an embarrassingly long lived bug). Lots of other stuff to love in the game too, do not get me wrong.
So yeah...TMNT is more of a mixed bag to me. I LOVE parts of it, but strongly dislike others. I am hoping Dwight revisits it with a code clean-up along the lines of what he did with GoT. There is a great game hiding in there, but it's bogged down a bit right now with some odd code choices. So far the good still outweighs the bad for me and keeps it in the collection, but that might not last forever. Stranger Things doesn't have quite the highs for me that TMNT does, but it also doesn't have anything near the lows. If absolutely forced to pick, Stranger Things would win my vote.