OP here, so I'm of course biased but I'm actually also a fan of the Tilt team :)
Overall, Garden has a broader focus, an added emphasis on testing, as well as capturing all the relationships between your services. The goal is for you to be able to use the same tools and workflows across development and testing/CI, and (as of our new release) the same environment as well.
For an apples-to-apples with Tilt, Garden does a lot of the same things when developing locally, but Garden additionally covers unit+integ testing and allows you to define all your bootstrapping workflows (e.g. migrations etc.). Tilt has a really good UI for local builds+deploys, but you wouldn't currently use it to run tests, or work with remote clusters.
The key thing we're adding in this release is not just an ability to work with remote clusters, but to do it _really fast_. Even faster than locally sometimes, because you can allocate more resources and share build+test caches with your team and your CI.
Plus—and this is more important for the long-run—Garden is designed to be pluggable to work with more platforms than Kubernetes. So you'll be able to use serverless, custom/legacy platforms etc. with the same tools when we release the plugin SDK later this summer.
