You've got a couple of really good answers, but there's another really important dimension: popularity. Choosing a popular language allows you to work with others, to hire people, to pass your project on, to allow your github repos to get popular, et cetera.

For that reason, I don't believe that Nim & D offer enough advantages and differences over Rust & Go that it outweighs their lack of popularity and hipness.

Of course others will say the same thing about Rust & Go: C, C++ and Java are much more popular and do currently fill basically the same niches as Rust & Go. You have to draw the line somewhere. It just happens that my line is drawn above Nim & D, but below Rust & Go.

Nim & D look like really awesome languages, though. They're just one killer project away from being popular, the way that Rails made Ruby popular. I would not be particularly upset if the situation reversed, if Nim & D got popular while Rust & Go got unpopular.