Here's the deal with traffic:
If you get an ADS-B receiver only (SkyRadar, Stratus, Clarity when it ships) then you really don't have traffic over ADS-B. Period. In fact, Stratus doesn't even try and pass traffic up to the iPad because they believe it to be unreliable at best and dangerous at worst.
The reason for this is the ADS-B ground stations don't pass traffic up to you unless you are broadcasting your position. Now, ADS-B is a broadcast, so you can hear any traffic that is transmitted, but the traffic sent out is only for things near the airplanes it knows about. So you might see 4 planes 14 miles ahead of you and think that you have good traffic coverage, but the reality is that the ground station is sending out traffic only for those planes far away and the airplane 1/2 mile away from you isn't being sent at all.
You can theoretically hear other ADS-B out planes, but only on the UAT frequency, not MODE-S transponders. Which is like 100 planes in all of the USA.
The world changes if you are transmitting your position in an ADS-B compliant way. With a certified UAT transmitter or a certified MODE-S + ES transponder, then the ground station knows you are there and gives you the most complete traffic picture it can.
In many cases, the ADS-B ground station will have access to non-ADS-B planes (the mode C guys) and will include that data in traffic it sends to you.
FIS-B is basically weather and TFR's, and that's broadcast all the time to all planes, even if you aren't transmitting. It includes no traffic data at all.
--Ian Jordan
Dynon Avionics