I think both of those focus on ingress? I suppose you could just create your CA with cert manager and manually issue cert requests, but securing in-cluster traffic (automagically) will need some other moving piece, like a sidecar proxy that the service meshes use.
As far as a service mesh, check out Linkerd! I find Istio much harder to setup and manage. Linkerd is super simple and has always worked pretty much out of the box for me.
One thing to keep in mind is that Linkerd is pretty much strictly k8s only one while Istio and Consul Connect have first-class support for out-of-cluster services as well as e.g. Nomad. Relying on linkerd digs you waaay deeper into k8s lock-in.
This may be fully acceptable for you, but should not be glossed over.
From my experience, linkerd had the most seamless deployment to get to the most feature-complete out-of-the box experience with monitoring etc. But as it goes with these things there’s a much bigger amortized cost in terms of magic to unwind if you need to integrate it.
