Well firstly this post prob belongs in the chaos section. Not the army list section. Secondly, as a veteran 40k player of about 7 years, I always find myself coming back to the different flavors of chaos. Chaos has all the rock hard defense of regular space marines but with daemons, more units, veteren skills, etc. In a choice between Tau and Chaos you have to ask yourself a few questions. If you want a straight forward army with not too much (but a decent amount) of strategy involved, go with Tau. Pretty much all strategies for tau consist of shooting an army till it's dead.
Chaos on the other hand, can be played tons of different ways. You can have a super-fast army (mine), you can have an assault oriented army, a tough defensive army, or like tau, a super shooty army. And even then you have lots of choices. For example, if you wanted a shooty army similar to to the crushing firepower of tau, you should go with either Iron Warriors, or Emperors Children. But, although both these armies can lay down a withering amount of firepower, both have a great potential to have assault oriented armies as well. IW can have bikes, or raptors, or cc-troop squads, and EC can have daemons, cc-squads, possesed, etc. Also, most EC armies are a mix of shooting and assault. Because even if you deck your army out with noise weapons the basic marine is still quite tough, and will strike 1st against most opponents. Guys getting too close to your gunns? Summon Daemons!
Another important consideration is, do you like the fluff (background story) of the army? Do you want to go parading around the galaxy in the name of the greater good? Or do you want to crush everything in your way in the name of the all powerful chaos gods? I suggest you read some of the tactics and articles here at LO. And maybe get your hands on some of the index astarted books (or white dwarfs) to read up on the storlines behind the different chaos unit.
Hope that helped, and good luck with your decision! 8)
