I believe Dimps has worked mainly on handheld Sonic games, in which case they've done a great job; the only exception, apparently, is the 3DS version of Lost World (I've never played it myself, but I hear it's tough enough to separate boys from men). Meanwhile, Sonic Team's quite obviously lost their touch since the mid-2000s, except for Unleashed.
Both devs worked on Colors and Generations (Sonic Team on the console versions and Dimps, as usual on the handheld versions). While the console versions are generally the better ones, Colors' DS version has more characters, the Chaos Emeralds, an extra boss fight, and arguably a better story. In contrast, Generations' 3DS version barely even holds a candle to the console one; even then, however, it still gets points for the great soundtrack (most notably the Big Arms remix) and having the definitive version of the Time Eater boss fight.
So, which do you think does Sonic games better?