The "3-wire" are the newest, and only work with a Mylaps decoder that has the Rc4 software.

The "2-wire" is the hybrid model, and works with the older RC3 software and the newer RC4.

You will find that the price is a little more for the "2-wire" as it is covers more decoder possibilities.

You kinda need to know what firmware the tracks you are running at have on their decoders, or just play it safe and get the "2-wire"