Well, when I was picking my 2nd army it was a toss up between Tau and Orks, in the end I went with Tau, and while there were a few factors that steered me towards the path of the greater good, I could've easily went the other way, the Ork models look great and they offer some great conversion possibilities.
In the end it came down to the Orks unpredictability. While it can make for a fun game it could also be frustrating, and since I am more of a controlled and coordinated commander, a rampaging greenskin horde didn't appeal to me as much as the cool and calm efficiency of the Tau.
Having said that, I've seen these new Orks in action and they can be devastating if handled properly, but I think Tau really are the essential shooters army.
As for 'Nids, I despise these fuckers, they are easily one of my most common and most hated opponents, the other being Eldar. I have suffered terrible losses at the hands of these armies, time and time again, but it's so satisfying to wipe the bastards off the table when things are going good.