Well, to me it comes down to 2 different things. The flavour of the faction, and how well I personally play with them. On that front, I love the Orks, Imperial Guard, and Tau for their flavour, but I play more in line with the Eldar/Dark Eldar or Tau - long ranged, fast combat. Sure the Eldar can mix it up in melee, and I love the specialism their units have, but I'm always more comfortable with my enemy at range than up close.
So I guess, given they're the only race I like both fluff wise and combat wise, I'd side with the Tau.