DOT5 is silicon brake fluid, and is not compatible with other types. Though many benefits were originaly claimed for it, there were also many disadvantages and it's not very popular now. DOT5.1 was introduced as a high specification mineral based fluid that is compatible with the other non-silicon fluids, e.g. DOT4.
Basicaly use whatever you car manufacturer recommends, but you should be perfectly fine using DOT5.1.