This is a tough one, because while they are both incredibly deep games with great themes, they couldn't be more different when it comes to how they play. I owned both at one time. I love TSPP, but it's gone (though I'd happily own one again for the right price). TWD isn't going anywhere, and even with a GB Premium in the house, I think I still play my TWD Pro more than any other machine. It's just a badass game.
If you have a family and want a fun, deep game that the kids will also be able to enjoy: go with TSPP. If you're looking for an adrenaline rush, get TWD, turn off the lights, load up Cleland's R-rated Pinball Browser sound hack, crank the volume, and get immersed in the zombie apocalypse.
No matter which one you go with, you win.