The nice thing about using separate clusters is that you can build them out in chunks. Build X new capacity in your main datacenter as a new cluster, and X new capacity in a different datacenter, and replicate. Need more redundant capacity? Build Y new capacity in your main datacenter, and Y new capacity in a different datacenter, not even necessarily the same backup datacenter as before. You end up with one main non-redundant cluster, and a bunch of smaller redundant clusters spread over one or more additional datacenters.

If you're really lucky, you siphon off customers from the non-redundant service for this at the same or faster rate as they are signing up for the non-redundant service, allowing you to not have to build that out much for a short while.