What's your mission?
David,
First my disclosure statement: I work at FreeFlight Systems in Irving. As stated in my signature block, I was building a 7A, but the kit, Reserve Duty, and work took too much away from the family so I sold the kit and am now looking at completed projects.
When I'm talking to folks about ADS-B, I first ask, "what is your mission?" If you are flying to Canada or Mexico, then you have to equip with a 1090MHz, transponder based solution. If you are not, then you can equip with a Universal Access Transceiver (UAT) which operates on 978MHz.
If 1090MHz is the path you need to go, then the transponder must be a Mode S/Extended Squitter (ES) type transponder. Just Mode S will not be sufficient. Several manufacturers have been mentioned here, but to rehash, Trig, L-3, Garmin, Dynon, FreeFlight, and several others offer a compliant transponder. The trick is the transponder must be "paired" with an ADS-B compliant GPS or one that meets the required performance as defined in the TSO. The NXT-9000 actually contains a GPS inside the unit, but all the rest require a separate GPS. Systems such as the Garmin 430W/530W, the FreeFlight 1201, and the Trig TN70 are examples of TSO's GPS devices you may use with the others.
If you are not going outside the US, then UAT is a viable option for you. The UAT is typically a complete solution in one box and most offer both ADS-B Out as well as ADS-B In in the same unit. Current key suppliers are Garmin, L-3, FreeFlight, and NavWorx. You will need to check with GRT regarding ADS-B In information on the Sport, but both FreeFlight and Garmin offer options which will display ADS-B In on your iPad using ForeFlight as well as other popular apps.
I hope this helps.
Mikey