Ok I can chime in here, I have owned both and do own 3 Will Vincent St 70s. one in triode. It depends on speakers and preamp, both are great amps, here is my analogy, as a football fan, the MC275 is like Jim Brown, one of the all time greats
The restored St70 is like Gale Sayers, also one of the all time greats, but boy do they run the pigskin differently.
With a proper and preferred tube preamp, and dynamic speakers, the MC275 has muscle and power and goes deeper, just like Jim brown with my Khorns, I would choose the ST70 every time, finesse, cut , dance and clarity with a run to the end zone, just like Gale Sayers.