[de_]

We measured both compressors filling 4 air tanks a) from empty and b) from 100 psi to 120 (more realistic in a match: compressor comes on at ~100 psi.

In both cases the Thomas was noticably faster. The 100->120 test was 28 secs versus 39 seconds. You might ask how that is possible when the Viair is rated at 0.34 cf/m at 100 psi versus the Thomas at 0.24 cf/m at 100 psi (at 13.2v). You would hope the difference would be there (scaled down) at 12v operating voltage. Clearly the Viair either falls apart at 12v or they have a different way of rating their compressors.
Of course the Viair is way lighter, way smaller profile for limited space robots but does run hotter (but just as you don't touch motors immediately after a match, don't touch the compressor). I have not heard of any reliability complaints.