The answer is: there is no difference.
What is OBD-II? OBD-II is an acronym for On-Board Diagnostic II (the second generation of On-Board Diagnostic) and often written as either OBD2 or OBDII.
It seems that different corporations used different terminology. The items like connectors, cables or scan tools have no standard of writing except of the OBDii connector should be called J1962.
If you look at some search results on MSN, Google, Yahoo or ASK, the most popular one is OBDii. Personally, I like more the OBD2 term.
OBD-II are well-known among car enthusiasts, so there's no need to be formal when talking about them.
