I've had both. Still have my TSPP, which was minty when I got it with only about 1200 plays. It ain't leaving, but it's the sort of game that my brain is wired for.
IF you want to get lost in the journey of the game and stop at various points around Springfield, and always feel like you are just scratching the surface, then it's TSPP. It is so enormously deep and complicated that in most cases you will spend a whole game trying to just get one part of it completed. Maybe get to AI, maybe complete secret stash, maybe get all the treehouse awards. There are literally dozens of ways to approach this game when you step up to start it. Sometimes I stand there with the first ball ready to plunge, too overwhelmed with the thought of the choices before me to even start.
But I love this and this is THE best machine for this type of play.
TWD is tight, like SM. Just beautifully executed code with a great build up to the bigger modes. Games will be shorter and will feel like a fight, like IM. TWD will obviously be newer and snappier, but you appear to have some older games in your collection so this probably won't be an issue for you.
Themes are obviously totally different and this is where your choice may actually be made. TWD really does freak little kids out.
Both are no doubt top shelf games and you really are going to enjoy both. But if I had to choose, and I have, it is TSPP.