Unipivot vs Linear Tracking

Alun, the problem with many air bearing tonearms is that the lateral tracking mass is much greater than the relatively normal vertical tracking mass.

This causes the cantilever of many cartridges to 'walk' back and forth as it produces all the forces needed to move the excessive mass of the arm toward the center of the LP as it spins. The result is that often the tracking error caused by the cantilever bending is much more pronounced that that of a radial tracking arm. This can be reduced by using a cartridge that has less compliance but you run into troubles with the effective mass vs the mechanical arm resonance (which is usually between 7Hz - 12Hz for best results).

A linear tracking arm that relies on a servo to move the rear tone arm will have a much lower tracking error and no need for a low-compliance cartridge. The Rabco was one of the few arms to use a system like that, but otherwise was a terrible arm. The 1960s for the most part was not a good time for tone arm designs :)

I only been using a linear tracking arm for a few months before then I had a silver wired SME 4 and to be honest the Terminator is just in a different class in every department. It has no hiss coming through the carrier
I am now totally convinced that this is the way to go to get the best out of your turntable

A linear tracker is theoretically perfect at tracking whereas with a pivoted arm that is impossible. That ends the whole debate. Whether or not one can hear the minute tracking errors is a whole different debate.  
And it's tiresome reading all the miss information about linear's or more specifically the ET2. It has great bass when set up properly with a proper air supply and it does not require constant fiddling. 

AL, The operative word in your first sentence is "theoretically".  Yes, an SL tonearm is theoretically perfect, but it is never actually perfect, which is one reason why the SL type does not completely dominate the high end of the market. The other major reason, IMO, is the need for a pump, the problems that come with that (noise, moisture in the system, etc, etc), and the problems associated with trying to achieve elusive perfection.  But I would agree that among SL tonearms, the ET2 is clearly a winner, along with a few others.