The answer to your paranthetical depends a bit on how much detail you care for. Both devices are types of resistive crosspoint memories. In XP, the resistor is a chalcogenide that has a low resistance polycrystal line state, and a high resistance amorphous state. These are tuned by the program pulse length. In TiO2, the mechanism is (presumably, perhaps debatebly) filamentary bridges between electrode, which can be eliminated by reversing the voltage.

So on one level, both are resistive memories. On another, we are comparing a bulk phase transition driven by pulse control to a filamentary transition driven by voltage.

Finally, the more important difference is the selector. PCM devices can use a chalcogenide based threshold switch. That is the OTS on the XP diagrams. That selector is the key to functional, dense memory arrays, and the existance of a matched selector is what makes PCM functional. I haven't seen what the matched selector for memristors is.

Worth reading recent material from Crossbar on these topics.