It depends on your interests, how much you want to pay, and what your play style is.
Tau for instance are easier to paint, and you don't need as many models for a playable or competitive army. They tend to be shooty, and fast. Battle suits move blow up a unit then move out of range. Most tau weapons are long ranged, so you can shoot from far away without worrying about getting shot up.
Guard are a pain to paint well though a simple paint job is a piece of cake, however you need models by the bucket load, and that is expensive and time consuming. They are however, a lot of fun to make, and you can use practically anything in a guard army as long as it looks guardy. Most armies I've seen have converted at least a couple models and the majority convert more than that.
The guard are slower than the Tau, their standard infantry weapon's primary use is entertaining kittens, and large amounts of infantry(or tanks) are a must. The guard are however, a tread heads dream. They have a vast selection of tanks and artillery, and boast quite a few variants of several vehicles. Their infantry are mediocre except in large quantities. With a 5+ armor save and terrible stats, individual guard don't last long, but big groups can mess up an enemies day. A big group of people use veterans mounted in chimeras and valkyries, but it is prohibitively expensive as far as I'm concerned.
Hope this helps.