I am wondering if there is an inherent superiority of either the 6SN7 or 12AU7
Yes there is, 6SN7 is more linear, has more power and has better sound called big tone than the 12AU7.
Tech differences are many, 6SN7 can handle 450V without any problems, but 12AU7 should not have more than 330V. 6SN7 can handle anode losses of 5W per tube, but 12AU7 is max 2.75W.
12AU7 can handle half the glow current that 6SN7 must have. Gain, internal resistance and slope are not that different, but perhaps the most important thing is that 6SN7 tubes give far lower distortion than 12AU7.
Morgan Jones has a very OK and nice overview of these tubes in the book Vacuum Tube Amplifiers and his conclusion is clear: tubes in the 6SN7 class and equivalents are far better than 12AU7.
The advantage with 12AU7 is quite clear that it is far smaller and uses less glow power than 6SN7. In smaller amplifiers that don't need as much output from the driver if it's used for that, the difference between the tubes is much smaller.
Some people probably think that the 12AU7 sounds better than the 6SN7, subjectively. That is personal taste, I prefer 6SN7 mainly to drive big triodes as 6C33.
