Both quite good games, well worth it if they can entertain you.
Devil Survivor is a fun tactical rpg; decent for what it is and holds plenty of character for the type of game. However it's not going to wow or amaze you in anyway with it's gameplay, characters, or story. It's really just well polished more of the same with Japanese flair.
It does some neat tricks, but never really stands out as more then tactical and a bit of standard RPGing you've always come to expect.
The World Ends With You is a strange beast all of it's own and personaly I what I think is the best made for handheld RPG. Diffrent style of combat, unique setting, focus on characters and building them in a way that is far more intresting then just basic grinding, and more.
Best part is you can actualy set up the difficulty per battle to best suit you and how you wish to play.
Best insight I found for The World Ends With You is the 1up RPG blog.
http://www.1up.com/do/blogEntry?bId=8996972&publicUserId=5725436
Several articles about the strong points the game has.
