6CA7EH or 6P3C-E in Mark III?

I'm basing it on failure rates and inconsistency...not just those reported on audiophile boards, but also by personal experience on my own equipment and amplifiers that I serviced while I was in the repair business.

No matter how good a particular tube type from a particular manufacturer might sound, my rule has been: one failure can be a fluke.  Two is a trend.  Three is disqualification.

So you are not talking about "close" meaning construction features or any such hard issues. You just wanted to say that old stock is better quality in your experience/opinion. Okay, fine. But that isn't all that helpful to a person trying to decide which power tube to use in a Mk III.

So I'll try to get back on topic by saying that the 6CA7 beam tetrode is tonally more like a 6550/KT88 than the pure pentode EL34 - regardless of when the tube was made. They share that common construction, and while as Roy said the 6CA7 doesn't have the power the larger tubes have, it can be a very satisfying listening experience.

One other thing to consider - 6P3S-E heaters draw about .6 amps per tube less than the EL34s. That lighter load allows the heater voltage to rise which MAY put the heater voltage higher than you'd like. It's worth checking if you are running 6L6/6P3S-E family tubes in your amp.