The word "advanced" is relative. While spark does support many of the new features of pairing to video endpoints, messaging etc, if you look at traditional UC perspective of features, HCS has a much larger set of core phone centric features plus contact center too.
HCS can be tied with Spark using Hybrid services too so it's not one or the other
Spark calling today purely in the cloud is targeted more at the 250 user market but can scale higher through hybrid services
Maybe one day there might be a question of overlap but today HCS can provide you the complete replacement for UCM on premise in an opex model and then Spark can tie into HCS for messaging features
