Really tough decision, both games are great. But I'll try to help.
If you played Ocarina of Time, Twilight Princess is like a slight better version of it, a few new things, more pretty... if you're a Zelda fan probably you like it (it's an excellent game), if not then stay away from it. You may think it's a little too similar to OoT, so if you want a new unique Zelda experience maybe you won't find it here.
Paper Mario: TTYD is different from SPM of Wii. It has more RPG elements and I think it's more funny, many players prefer it to SPM, but it's all about preference. It's a great game and if you liked SPM probably will love this one.
Other games would be Super Mario Galaxy, it's one of the best games of Wii (if not the best), if you liked Super Mario 64 just buy this! You won't be disappointed. It's really pretty and fun.
Still talking about 1p games, I think Resident Evil 4 is splendid. If you didn't play it on GC you should play it now, if you like adventure/shooter games there's no way you won't like RE4.
Super Mario Sluggers nobody outside Japan has played yet, so it's better to wait some feedback from other players before deciding to buy it. And it's a multiplayer game, if you're looking for a solid 1p experience it's better stick with the others.