I actually have had a cluster of Raspberry Pi 1Bs before this current cluster. Though the specs are very much similar to a Zero W, the performance wasn't great.

I agree that it would cost much less, but I would also get much less utility out of it. At this point, the cluster is doing quite a lot, even the site itself is hosted on it.

My point here is that having room for future utility at not much additional cost, would be best.