What made you turn away from javascript and into templates?
We've basically covered all those idioms in a few paragraphs. There's very little extra stuff to learn. Now if I may be blunt in return, I was converting the React RealWorld implementation to Svelte for the purposes of this post, and there were moments that I spat out my coffee at how absurd some of it was — twice as much code, with some truly bizarre (but idiomatic-to-React) constructs. It's all a matter of perspective and familiarity!
Performance-wise, you'll get great results with either framework. You mention debugging — Svelte creates useful sourcemaps, and the generated code is very readable anyway. Svelte has a few features you might find interesting (AFAIK Marko doesn't offer these, though I'm not intimately familiar with it):
* it can compile directly to custom elements * your styles are scoped to the component * declarative transitions * built-in global store (think Redux, but zero boilerplate) * useful element bindings (for e.g. customisable media players https://svelte.technology/repl?example=binding-media-element...) * computed properties. these are a lifesaver when you're doing a lot of complex reactive stuff
and so on. Also, I don't think Marko has an equivalent of Sapper.
Finally, while Marko is slimmer than the likes or React or Vue, there's still a runtime library you need to include on your pages. A typical Sapper page is about the same size as Marko by itself, before you've added any app code.