Also, both tools will fit the spindle, the difference is in the location and such of the fingers in the tool changer, as the location is different on the tool holders. CAT40(Caterpillar designed I believe) is the most common used. BT40, metric, is the European standard.
Both work the same, but you may find more tooling for the CAT40 machines.
The spindles are identical from what I know.