THe 825 and 655 are essentially the same HW. Just the 825 has a 5Ghz radio. If theres only a $10 difference then I would spring for the 825. I'm currently using it right now as my main router. Solid router. Great WiFi.
Even if you don't have 5Ghz supporting, you can turn this radio OFF until the day you get something that supports it.
If your looking for a good cable modem if your ISP is cable, I highly recommend Motorola SB 6120, 6121 or 6141 modems. Great modems and work well with DLink routers.
QoS is for maintaining good data bandwidths while multiple devices are online at the same time.