The subscription is for their own application. It's an itune app like foreflight or wingx that It uses seattle avionics charts. With the ifly you don't need their subscription.
I can't really speak as an expert of the dual band. I definitely suggest doing some research. From as best as I can tell, and I am not expert, the commecial traffic above FL180 will be using the 1090mhz. Even when they are below FL180 they will still use that frequency. The way I look at it, TIS is a crap shoot. Having dual band might increase the odds of actually seeing something. Given that the price is the same between stratus and the skyradar dualband AND the skayradar works today with the ifly, it seemed a no brainer:)