Tau are cool looking (well, fire-warriors are >.>), and really good shooty, but unless you want to use one of their borderline-cheesey tactics (fish of fury or mass JSJ), you will have a fairly rough time winning games against skilled opponents. Line tau are just too easy to kill by simply getting into combat.
On the other hand, Tyranids have an obscene number of viable builds. And they can be shooty, slashy, or balanced, as well as full of big things, full of little things, or a mix of the two. They can be awesome at close range shootyness (about even with battle sisters for points, but without needing transports), and follow it with charges that, even from 6 point gaunts, can be devestating.
Only you can make the decision on which army to play, however, so other than that, I can't really help you.