None will disappoint you really but I have to give my vote to the DS.
It has a much bigger library and carrying it around is pretty easy (it's light and not very bulky) and it's screen are protected by it's own casing when closed and the casing is very VERY resistance (if there are any scratches or dents on mine, they are not noticeable at all).
It has a lot of pick and play games that work great for the console but it also has a lot of deep games like Castlevania and Phoenix Wright (my two favorite series on the DS) not to mention games like Puzzle Quest, Tetris, Meteos, Mario Kart, Metroid Prime Hunters, Mario & Luigi Partners in Time, Dementium The Ward, Zelda Phantom Hourglass, New Super Mario Bros., The World Ends With You, Final Fantasy XII Revenant Wings, Geometry Wars Galaxies, Space Invaders Extreme, Nanostray 2, Elite Beat Agents, Advance Wars, Civilization Revolution (exact gameplay and content as console versions, just downgraded audio and graphics and no civilopedia), Contra 4, Final Fantasy Tactics A2, Ninja Gaiden Dragon Sword, Trauma Center 1/2, Warioware Touched, Hotel Dusk and who knows what else I forgot, oh and Pokemon Diamond, Crystal Chronicles and Dragon Quest Joker.
That being said, there really isn't much to look forward for the DS, I'm eagerly waiting for a new Ace Attorney game (though nothing has been announced) and Castlevania Order of Ecclesia later this year as well as the Pokemon Diamond/Pearl remix but aside from that, there really are no titles in the horizon maybe Rhythm Heaven if you are into music games (I sure am). Oh there's that N+ game coming out as well as a new GTA for the DS and that Prince of Persia platformer which looked pretty cool in the gameplay trailer.