If you have a USB3 card reader (and USB3 on your computer of course) the faster cards will significantly speed up downloading to your computer, less than 5 minutes for a full 64Gb card. Pentax recommend Class 6 cards or higher, if you shoot HD video you definately need at least Class 6. I use these from a reliable UK supplier
SANDISK 16GB SDHC VIDEO SD CLASS 10 EXTREME MEMORY CARD | eBay
To get these download speeds, the 8Gb is not that much cheaper. UHS Class 1 cards are even faster to download and offer 'future proofing' as cameras and computers will inevitably get faster.
Chris