The higher the DOT rating, the more quickly the fluid will absorb water from the atmosphere and the more often you have to change it. A DOT 4 fluid, despite having much higher boiling performance ratings than DOT 3, will have the nearly the same boiling point when saturated with just 3.7 percent water. 3.7 percent water is what's called the wet boiling point on the bottle.
DOT 4 fluids will be fine for everything but a dedicated track car. Believe it or not, when the rotor is glowing red and over the 1200 degrees, the fluid rarely overcomes 400 degrees. Most good DOT 4 fluids are rated at around 500deg +/-, dry boiling point.
So most people can just use DOT 4, (I use Amsoil personally), and after the occasional track day make sure to do a service bleed. Save the money if you only go to the track occasionally.
And remember guys, once a brake fluid bottle is opened you should use it because it will be rapidly absorbing water when on the shelf.