What is the difference between H and M xD Cards?

I'm looking to buy an xD Memory Card for my new digital Camera. Particularly i was looking at Olympus cards (since my camera is Olympus too), but got stuck: found 2 cards, both 1 GB, but one is H xD and the other is M xD.... What's the difference?? (except for price =P)

I couldn't find any difference at all between the two xd memory cards even when I used to work in a photographic company and sell them. The only two things i know about them are:

1. they are high speed high capacity
2. they don't work in some old fuji cameras.