Both, but unfortunately still mostly using Celery 3.2. Never had any problems with zombie tasks, since that's related to how you're storing tasks and queues and RabbitMQ is excellent at that. Zombie tasks are never a problem when using RabbitMQ.
Especially never had problems with zombie workers. That wouldn't be a problem with your choice of queue library, more an infra problem.
Interesting. At the moment I'm noticing some zombie workers popping up here and there with RQ and was thinking of migrating soon to Celery or Dramatiq. I think I'll experiment a bit and decide between both (but Celery seems like the more popular option, so I'm a bit biased for going for that).