I feel like its more dependent on how you play and what you're up against. If you can get in, take someone out, and get out, then R might be more useful. If you are seiging/poking more, W might be more useful.
It used to be (not sure if it still is) that when you hit things with more than one W once you upgraded it, you could get the heal more than once, so if you were doing a lot of melee dueling, that double or triple heal was very nice. I think that got removed some time ago, but I don't really play Kha so I don't pay much attention to his changes.
It also depends on how well you can hit multiple things with w. If you spam it and miss most of the time (or only hit 1 person), you're just wasting mana.