12BH7 tubes are physically "taller" than 12AU7 tubes and are probably more durable in their ability to handle voltages. I have used both types on the VTA lower gain board and I don't really have a preference for the center (initial voltage amplifier for both channels) position. For the two side phase splitter/inverter tubes (IMHO) a 12BH7 may be a better choice because of its ability to swing voltages better than a 12AU7. If you want to mix the two tubes on the lower gain board, the best mix is probably a 12AU7 in the center and two 12BH7 tubes in the two side positions. Depending on your hearing, your speakers and your associated audio gear you may find one combination better than the other.
On either the VTA standard gain 12AT7 board or the VTA lower gain 12AU7/12BH7 board, the center tube has the most influence on the sound of the amp. On the lower gain board either a Mullard CV4003 or an RCA clear top are two of the better choices for the center voltage amplifier.