The class 6 rating is for the minimum transfer speed that the card will deliver. This usually means the WRITE speed of the SDHC cards, so a card that has a Class 6 rating means it should write at a speed of 6 MB per second or more. The Read speed can vary from each manufacturer, but it is usually 2 to 5 times as fast as the write speed. The write speed is the main bottleneck in Windows OS, both on SDHC and the internal
SSD cards. The price of class 6 is just a few dollars more than the slower class 4 and 2, so nobody should try to save $5 dollars and suffer slow speed for the rest of your Mini's operation life.
I got a few A-Data 16 GB SDHC for $36 each, with free shipping from Supermediastore.com last month. That's the going rate among most online sellers that are selling 16GB cards. Whatever you buy, just stick with a brand you feel comfortable with when you need support or warranty service. The 16GB seems to give the best byte per dollar. The 32 GB cards' prices still high, at about 3 to 4 times as much as the 16GB.