There are basically two standards for ADS-B Out:
978MHz (UAT) Out is acceptable for aircraft operating below FL180.
1090MHz (ES) Out is required for aircraft operating above FL180.
As far as I've heard, 1090MHz will be the adopted standard internationally as well.
As for traffic detection with Stratus, it is my assumption that either Stratus box will receive and display air-to-air UAT traffic, because that is how the system is supposed to work. However, I suspect that most of the present aircraft transmitting ADS-B Out are commercial operators transmitting on 1090MHz, not UAT. Unless you have the dual-band receiver (Stratus 2), you won't see 1090MHz traffic except via ADS-R (the rebroadcast of that traffic from a ground station).
A summary of my understanding on the Stratus capabilities:
Stratus 1 will receive on 978MHz for ADS-R (1090 traffic rebroadcast), TIS-B (Mode A/C secondary target rebroadcast), UAT air-to-air traffic, and FIS-B (weather information).
Stratus 2 will receive on 978MHz and 1090MHz, which includes all capabilities from Stratus 1, plus the ability to receive air-to-air traffic from aircraft transmitting ADS-B Out on 1090MHz. Stratus 2 also includes AHRS, for those who want a non-certified horizon backup using Appareo's standalone app designed for this purpose. I suspect that ForeFlight may integrate the AHRS capability of Stratus 2 into a synthetic vision display in the future.
JKG