Author of Kobweb here. Note that Kobweb is not Wasm, it's Compose HTML. I don't think that will matter to you, unless the hackathon is about what you can do with Wasm :)
I'm biased but I'd suggest Compose HTML for a hackathon in most cases today, since there are a lot of resources online about traditional website development while Wasm is still very new.
If you go with Kobweb, feel free to ask questions in the #kobweb channel. Good luck!