I have no experience with calling c++ from Nim but - you might be better off with C++ if you are planning to heavily depend on other c++ libraries. Otherwise all comes down to your taste.
Araq, I know you are constrained by time but I'd love to see additional information/examples in the importCpp-pragma section. Perhaps examples of interfacing with more general libraries such as std or boost - to get familiar with Nim's usage of pragmas. Also it is not clear to me what are current limitations of interfacing with c++.
From my perspective (learning Nim for game development, 10+ years of experience with dynamic languages)
Nim attracts lots of game developers, and there are already lots of wrappers/bindings for popular game libraries/frameworks.
Also, with c2nim, writing bindings for c/c++ libraries is possible with not much effort (so I've been told.)
Rust has a totally different direction than Nim. Nim has memory safety with GC, and it's GC is suitable for games.(You can specify when will GC run and how long) Rust has memory safety without GC, and puts the burden on the developer with it's lifetimes/borrows etc.
Rust is more popular, due to it's backing from Mozilla, and probably has more stuff readily available due to this. Nim is getting there also, slowly and steadily.
I'd say give Nim a try. Grab the latest compiler from github, get opengl/sdl/sfml libraries, and start hacking stuff. It'll be fun.