I'm very, very happy with my DI-604 so I thought I would call D-Link. They answered the phone promptly and had all of the info I needed at their fingertips.
They recommended their
D-Link DFL-CP310
router but unfortunately, although it has a backup WAN port, it can only use one WAN port at a time. It also supports QoS and Static Routing.
So thumbs down on this product. If you don't need the dual WAN then it might be a pretty good, albeit expensive, choice.