AFAIK AssemblyScript was born out of dissatisfaction with some of the design choices in TurboScript. Not sure what the exact differences are anymore, looks like TurboScript is moving to binaryen now as well. I would personally go with AssemblyScript in a heartbeat just because I know that the dcode guy makes some great modules.

It doesn't decompile from wasm, and I don't think it ever will. You could maybe use SourceMaps to go back when support for that is added.