I went with the Trig TT21.
Here in
Australia the Class 2 TT21 is sufficient or operations below 15000ft for aircraft with normal cruizing speeds below 170KT (referred to as ADS-B Class B0).
(See:
http://www.comlaw.gov.au/Details/F2009L04631)
I don't have the GPS yet though.
In Australia we have to be fully compliant by Feb 2017, however, any new aircraft to the register after Feb 2014 have to be equipped with a compliant transponder. Also, any transponder replacement on existing aircraft have to done with be compliant transponders.
I will fly my aircraft later this year (hopefully) but since you don't need to be fully compliant, just have a compliant transponder, I am waiting with the purchase of the GPS till 2017.
Hopefully the prices will have dropped by then it is expected that they will drop now the U.S. deadline is also getting in sight.
This document exaplains the above in detail:
http://www.airservicesaustralia.com/wp- ... 8MAR12.pdfAgain, for Ozzie builders only, the U.S. seems to be going one step further requiring a Class 1 Transponder for all.