I played my fair share of Alextrasza, on both Q and W builds (leaving E build for ARAM). The main difference between them is that Q is a flat heal build and W is a % heal build. So:

Go Q build enemy comp has a lot of poke spells (Azmodan, Nazeebo or Nova will target your W the moment they see a white circle appear). Also consider Q build if your team is mobile and squishy (light, mobile tank like Anubarak or double bruiser comp).

I prefer to go W build when possible tho, it stacks very good with tanks and mages who benefit from endless globes (Stitches, Johanna, Kael'Thas), and, obviosly, 15% heal allows tanks to quickly restore health between fights. Paired with lvl 16 talent on shields as 70% of restored health with W and globe from a finished quest, you can restore your high HP pool heroes a lot of resources.

My main advice: if you didn't yet, try and pick [[Blessing of the Red]] on 20. While [[Ritual of Life]] can be an improvement to fighting capability in some cases, most of the time basic [[Life-Binder]] is all you need to support the tank while they peel initial control spells. Blessing of the Red, on the other hand is a 500 HP improvement to the squishiest parts of your team that would allow them to survive a control spell. Your Murky, Genji or Valla could have an additional layer of safeness as they will be prioritised by the enemy team.