If you take a look at this thread
http://www.oople.com/forums/showthread.php?t=121966 it will tell you what clubs are using what type of decoder.
In short:
If the club is using a RC2 or RC3 decoder then all PT will work EXCEPT the MyLaps Purcy (also referred to as the RC4 Transponder).
If the club is using the RC4 decoder then all PT will work EXCEPT the MRT.
My club has done a comparison with the MRT, old square AMB PT, the Harry and the Purcy on both an RC3 and RC4 decoder and I can say first hand what I have written above.
If you are in the market for a new PT the best one to go for (in my opinion) is the MyLaps Harry (also referred to as the RC4 Hybrid Transponder) as you can be sure that this will work on RC2, RC3 and RC4 decoders so it doesn't matter where you race
