I think they are similar but a little different. From Dagger's website "Dagger is a programmable CI/CD engine that runs your pipelines in containers" Kargo is a tool that works with your existing CI and CD (Argo CD) to help promote your code from one stage (Dev, UAT, QA, Prod, etc) to the next.
Kargo has native support for Argo CD today, but one of it's goals is to be agnostic and could probably work with Dagger or other tools in the future.
I'm looking forward to seeing support for other GitOps tooling. Argo CD makes sense initially since this is by the same creators, but it would be nice to see agnostic tooling that can support other GitOps tools like Flux. I hope this is something that we will actually get to see and isn't just an empty promise.
Not much overlap. Dagger wasn't on my radar until now, but it reminds me of something else I used to work on and it frankly looks awesome!
But it looks to allow you to defines pipelines using your preferred imperative programming language -- which is awesome. I would totally use that for CI.
Kargo is really about divorcing CI and CD and creating something that's actually CD-focused and compatible with established GitOps principles.
Dagger looks more comparable to Argo Workflows: https://argoproj.github.io/argo-workflows/ That's the first of the Argo projects, which can run multi-step workflows within containers on Kubernetes.