BUK-M3 doubles the range from previous Buks, or HQ-16s from 50 to 70km to 130km. It also travels at Mach 4.6 so that is a pretty good interceptor --- the faster the better always for interception. Best yet, it has active guidance, which is a major departure from the SARH used in the previous Buks or the HQ-16.
Buks have a much bigger warhead than the S-350, about 70kg to 22kg. The warhead dropped in the 9M317 to 62kg, and I am not sure if the warhead is reduced to allow room for the active guidance system which means an emitter and battery, while keeping and increasing the same propellant weight.
I think this might be a better missile than the S-350 Vityaz due to its speed and punch, but its bigger and bulkier.
It maybe better than any current HQ-16 right now, although I think in terms of ground systems the HQ-16 might be better having larger ground radars. But in the future the HQ-16 can also be improved for greater range and an active guidance system. The main limitation of range for this missiles has been the SARH, where your effective range is the slant range of the missile target illumination radar, even if your missile has the ballistic capability to greatly exceed that range. The active guidance system --- putting the illumination right on the missile itself --- removes this chain that blocks the missile's flight potential. In fact one way a future HQ-16 can be better is to utilize the AESA based active homing system used on the PL-15 LRAAM.