If you are buying a large card, the speed differences for downloads could be meaningful. The camera has limitations since it has to process and write the image. Write speeds are often slower than download speeds. A fast card full of large files of images or video could be much better if you want fast downloads.
UDMA 6 is probably a pretty good answer - just get a large card. I think 32 GB is a starting point.
There are a lot of options on the second card. You could use an 8 GB Eye-Fi card for medium or small JPEGs, or a 16 GB card for video and large JPEG or overflow NEFs. I don't see a major need for the two cards to have the same features. But the larger file size means you'll need three times the storage of your current cards.