First playthough, both are very interesting. On Taris, there's plenty going on as you try to figure out where Bastila is and how you're going to escape the Sith blockade, even if you could find a ship. On Peragus, there's the murder mystery tied into the Harbinger (And every time I go to the engine level, I picture the Exile saying 'While we're here, maybe we should film a horror doc...' to lighten the tension).
However, the repeat playthroughs are when you start deciding on whether one is better than the other. And in the terms of replaying, Taris tops Peragus. On Taris, you can play things differently with the different subplots, ignoring some this time, doing them all another... On Peragus it's a straight shot with the only differences on each playthrough being dialogue. It's always going to be a dead station, HK-50 is always behind the murders, and the Harbinger is always lifeless.
So, given those terms, Taris is the better planet.