I wouldn't necessarily expect a USB 3 device to be 'more capable' than a USB2 device. The faster throughput of USB 3 could mean more channels in a given interface but the current crop of USB 2 interfaces can handle quite a few streams of audio in and out of the computer. Focusrite offers an interface that will do 18 inputs and 6 outputs over USB 2. (Others may offer more.. but I was on their site looking up the interface you mentioned so I took a quick peek)
USB 3 is fine technology but I wouldn't get too caught up looking for a USB 3 interface over a USB 2 interface... to be frank I'm not aware of any strictly USB 3 interfaces though, of course, the USB 2 interfaces are compatible with a USB 3 port on a computer. At present I'm noticing higher end interfaces supporting thunderbolt and/or Firewire instead of USB 3. Others may have more current news that I do.. but I've not heard of anyone offering a strictly USB 3 interface yet.
The Focusrite Saffire 6 USB interface that the vendor mentioned to you will certainly meet your needs and, with a price a right around $200, is a reasonably good value. Most interface manufacturers offer similar devices at a similar price point but you could spend as little as $60 for a Lexicon Alpha and still get the features you mentioned and compatibility with Audition. Likewise, you could spend a great deal more if you wanted to. The more costly interfaces will offer you a larger feature set, higher quality, higher sampling rates, more inputs and outputs.. or any combination!
Do you have a budget in mind? What are you recording? You have modest I/O needs now but do you expect those needs to grow?
Good luck!