Hi rinpoche, unless the M725 monos on sale are the S2 version, the M625 S2 that you are considering will deliver a much purer, more musical sound.
The original M725 of course will be more powerful than M625 S2, because each monoblock is equipped with its own 1200W SMPS and PFC unit.
Thus, unless sheer power / authority is a factor, I recommend you go for the M625 S2.
For the best of both worlds, there is now an M725 S2 in full production.
Saluti, Guido