My understanding is that SDHC (Secure Digital High Capacity) and MicroSDHC cards are essentially the same, except for the size. They make adapters for MicroSDHC cards to fit into mini and regular SDHC slots.
As far as the speed rating, I think a class 6 is a class 6. You're generally pay more for a fast (class 6) MicroSDHC card than you would for a regular class 6 SDHC. Higher capacity Micro cards are also generally more expensive than their regular-sized counterparts.
I usually buy Micro whenever I can just for more compatibility. Maybe I'm using it for computer storage right now, but down the road I might want a 16GB card in my cellphone instead (especially when 64 and 128GB become the norm).