I'd say none of the above. There have even been a few interesting projects done with Ocaml (mldonkey, unison), which is a pretty good choice for system-level programing and has a pretty fast compiler. Also a lot of people are using Erlang for it's supreme concurrency support and stability.

Which is why I don't doubt that there'll be quite a few people using Rust and/or Go. But I'm really not sure whether either of them will move beyond this tier into the PHP/Python/Ruby territory. A framework like Ruby would certanly help, and that can come out of nowhere. But I wonder whether the day of killer-feature frameworks aren't numbered, i.e. whether something like this can happen again.

So I think in the end it comes down to which of the two will do better in the Haskell/Ocaml/Erlang tier. Go has some rabid fanboys and quite a decent designer pedigree, good libraries and a bit of a headstart, whereas Mozilla is ripe with dumped applications and failed projects. So unless they're rewriting major parts of their infrastructure with Rust and actually release stuff, I'd bet on Go.