Nintendogs is more detailed, and entertaining. I love the Sims 2 Pets for the PC, but the DS versions just haven't been able to live up to that. Nintendogs is an amazing game and really uses the capabilities of the DS.
the sims 2 pets for ds gets VERY old and boring after about an hour of playing it, u dont build or choose the house, ur character just moves into it, and u can only make one character per game, theres very little furniture to choose from, no cheats, and ALL u do is take care of these peoples animals, its also got pretty bad graphics, but nintendogs is another story, I LOVE that game! its got great graphics and it never gets old!