I'm pretty sure there are several threads here on the WOC. The lambskin WOC is great - I've used mine several times and there are no marks or scratches. I haven't tried the mini but I can attest that the WOC holds a lot. It has several credit card slots, 2 long slots for cash/bills/receipts, a zip pocket for coins,etc, and a larger central compartment that can hold ipod/blackberry and a bit of make-up. The chain is a great length for wearing messenger style, and it offers the versatility of being worn as a shoulder bag or clutch as well. The only difference I think is that the WOC is more casual, though there are metallic versions that can be worn for evening.
Caviar is a pebbled type of leather that is quite hardy and needs minimal care.