I have TMNT, neighbor has STh (and I rented it for a month prior to him buying it). Major distinctions imo:
- STh is much more approachable from a gameplay standpoint. The shots are generally easier and drains are less freqent. That said, the flow of shots in TMNT is incredible when you're on, but it's less accessible for newer/less skilled players.
- TMNT is colorful, goofy, and high energy. STh is way more dark and heavy, and at times a bit sleepy.
- TMNT has more side objectives, and co-op is extra fun. Each turtle having different perks makes for great depth.
Personally I prefer TMNT, even if it is super brutal. I just like the overall tone and layout (have MM, so fan layout is covered), but it's not for the faint of heart. STh needs the Cleland sound package and is easier for a wider crowd (although dark for kids), but still lots of fun. Tough to go wrong with either.