I believe Rancher desktop has more of an out-of-the-box focus on Kubernetes, it uses Rancher's k3s and whatnot. Podman desktop treats k8s as more of an optional thing, e.g. you use Kind to create a cluster.

Of course there are also organizational differences, Podman seems to be more of a community led open-source project whereas Rancher is ultimately trying to sell enterprise kubernetes management software.

In my experience, k3s looks great on the surface but I ran into quite a few issues operating it in a single-node use case.

If I was looking into this today, I'd definitely explore Podman/Kind instead.