What is the difference between DOT brake fluids? I know that the two main categories are the mineral oil (Shimano, Magura) and the DOT type (Avid, Hayes).

But what is the difference between the DOT 3, DOT4, DOT 5.1?
Which one should I choose for average mountain biking?
From Wikipedia:

Part of the difference is boiling points:


               Dry boiling point   Wet boiling point
DOT 3       205 °C (401 °F)     140 °C (284 °F)
DOT 4       230 °C (446 °F)     155 °C (311 °F)
DOT 5       260 °C (500 °F)     180 °C (356 °F)
DOT 5.1     270 °C (518 °F)     190 °C (374 °F)


3, 4, and 5.1 are glycol-based, and must be changed periodically, as it absorbs water. 5 is silicone-based and does not need to be replaced.