These are tough question to answer because everyone has an opinion. What tube sounds good in one person's amp may not (because of their hearing, different speakers etc.) sound as good in someone else's amp.
1. 12AU7 (includes also 5963, 5814, 6189 and the Mullard CV4003) vs. 12BH7. Many like the 12BH7's because they are a physically taller tube than a 12AU7 but I don't hear any particular "sound quality advantage" to 12BH7's especially in the front (voltage amplifier) position of the M-125. In my own VTA M-125's I do use a GE 12BH7 in the rear phase splitter position. I use a Mullard CV4003 in the front position.
2. In general NOS tubes will usually last longer than current production tubes. Some newer production tubes sound pretty good. With respect to 12BH7's, the ElectroHarmonix 12BH7 is the only current production 12BH7 tube. You have more brand choices in modern production 12AU7 driver tubes.
3. You can still find NOS RCA, Sylvania or GE 5963, 5814 and 6189 tubes out there at reasonable prices. A lot of tube dudes do not know that these three tubes are 12AU7 "clones". Sometimes you have to weed out a noisy or microphonic tube but these three all work well in the VTA M-125's. Of the modern production driver tubes, I like the Russian made ElectroHarmonix tubes.
4. For well tested tubes get in touch with one of our forum members "jimmcshane". Jim is a private tube seller with an excellent reputation for providing quality tubes to customers.