I'm wielding a staff focus, [which doubles as a quarterstaff](https://twitter.com/mikemearls/status/509454576999612416); so, it seems that Polearm Master + War Caster allows a PC to cast spells as reactions to enemies that enter its reach. 

Similarly, Sentinel + War Caster allows a PC to cast spells as reactions, even when the enemy takes the Disengage action. 

Three questions:

1. Can I combine Polearm Master/Sentinel with War Caster to cast spells as opportunity attacks when an enemy enters my reach (PM) or when they Disengage from reach (Sentinel)?
2. If I want to maximise how often I can use my reaction to opportunity attack with spells, which is the better pick, Polearm Master or Sentinel? On the face of it, it seems like the former, but I've only started playing, so, not much experience yet.
3. If Polearm Master is superior (as described above), is there a way to compare the value of Sentinel's "stopping effect" (on a hit, you set the enemy's speed to 0) vs. the added damage? The added element of battlefield control can be quite valuable (though hard to quantify).