Ghost has more features, and tries to bundle more. Write.as/WriteFreely is much simpler (and less mature) project, but you are right it supports the Fediverse and it’s also written in Go which is much safer and faster.
Also, Ghost is apparently dropping support for non-oracle mysql which is a big thumbs down for many.
Allowing readers to send in article submissions (super useful!), or OAuth support may be another consideration towards Write.as