Well I have used the first three, and currently using Avydine TAS most of the time.
As you probably realise the first two operate in a very different way to Zeon; firstly they are active systems and secondly they have dual aerials. The way they process the information is also quite different.
Zeon warn in their manual that not all "transponding" aircraft will be seen - for example without an aerial underneath the aircraft in the circumstances they describe an aircraft may not be detected. Also Zeon relies on ground based (or other aerial based TS interogating transponders) so in theory if no one else is interogating other aircraft they will not be seen by Zeon. Since the vast majority of the UK is covered by radar I am not sure how much of a problem this is (albeit at low level it could be significant).
I have not used a Zeon linked to a moving map display so I cannot compare this with the first two. Without this feature there is obviously a huge difference between the systems in terms of your situational awareness of other aircraft. Displaying targets on a moving map gives an extraordinary improvement in where to look for traffic as does the ability to trend the height of the aircraft relative to you.
The first two systems are amazingly accurate. You are probably aware that theoretically they should not be used for "blind" traffic avoidance and the target should always be acquired visually. In reality with many 100 hours using TAS the aircraft has always been where indicated.
You will be aware that the first two systems are certified installations - in theory this should guarantee a certain integrity of the installation and the performance of the hardware.
Once you have flown with TAS it is difficult to fly without.
If money were no object I would opt for either of the first two options coupled to a large MFD.