Hi Jim,
I don't think it's so much a case of the latter, as that will simply come down to which is most electrically suitable in a particular circuit.
You'll have heard what I have, by mixing different valves (of the same type), from different manufacturers, simply as a result of the effects of their (varying) constructions, such as anode plate/mica/getter structure, how the valve concerned has been designed to help eliminate the effects of vibration, different pin/base materials, etc, and even the size and construction of the glass tube itself.
Just because, say a Mullard ECC83 and a Telefunken ECC83, perform the same electrical function, it doesn't necessarily mean that they will sound the same!
And if they don't sound the same, then their respective sonic signatures, just like with cables, will influence what you hear accordingly, which is why you can hit upon certain combinations that just 'work' (better than others), simply because they
combine to create a sound you like, and which works with (rather than against) how your system has been 'voiced' [as a result of the choices you've made to date, in that respect].
Hope that makes sense?
Marco.