I used Z10 'Leather in a Bottle' to treat my leather before I got stained/soiled. It is really good stuff, no residue at all, no jean stains.
I would assume the Z9 Leather Cleaner is just what it says, a non-abrassive cleaner for stains.
I'd say get and use both.
Sparkling Bronze 2011 X5 xDrive35d
Premium, Technology, Sport Activity w/ 20" 214's, Cold Weather, Premium Sound, Rear Climate, Multi-contour seats, Comfort Access, Roof Rails, Oyster Nevada leather