Scott,
The SkyGuardTWX is a larger "thicker box". Keep in mind, the SkyGuardTWX is a Transceiver and the SkyRadar is a Receiver. The SkyGuardTWX is one of the smallest Transceivers on the market. SkyRadar makes a great ADS-B Receiver, but it won't activate the full traffic like the SkyGuardTWX.
Also, for the antenna. They look the same, but according to Don at SkyGuardTWX, there is a difference and they get that specific modification for their antenna. The SkyGuardTWX antenna has a UTC 1-second timer inside it, where the SkyRadar does not. It's my understanding that they are not compatible. It will likely still show GPS position, but "could" affect the Transmit/Receive of the ADS-B data.
I hope this helps clear up an confusion or an questions.