For me iteration speed's a big selling point that (plus the fact that's easier to find contributors) might also be important for projects like these.
Rust seems to me far easier to learn and get going in due in major part to its incontrovertibly superior standard tooling.
I can’t see any place for any meaningful difference in iteration speed between the two, save that you may well have to iterate more in C++ due to memory safety bugs the compiler doesn’t catch.
As for finding contributors, I get the impression that Rust is considerably more accessible, and thus will increasingly find contributors more easily, as people that just love programming will actively choose to learn Rust far more often than C++. (For the current state of affairs, I think it’ll depend on what sort of contributor you’re looking for, in skill, industry, paidness, &c. Some segments will certainly go one way, and others certainly the other.)
With Rust, though, it's as if someone looked at C++ compilation times (not to mention resource requirements) and said, "I think we can find a way to make it worse."
But I still really like Crystal.