I really wouldn't consider it reasonable comparison, when one language has a sufficient compiler that is written in itself for most part, and the other still only compiles to C.
Also, I find one of your statements quite contradictory: by saying that Rust's compiler "is written in itself", you seem to imply that Nim is not. But this is not true: Nim's compiler is almost 100% pure Nim.
(Last but not least: to me it seems not true that Nim "only compiles to C", as it provides multiple backends. See here: http://nim-lang.org/backends.html .)
Generally, the LLVM route will be faster/more compact when compared to Clang. However, nim can just as easily use gcc, or MSVC, or tcc, which rust cannot. Especially with respect to tcc, this means a stand-alone all-inclusive nim compiler executable can quite easily be constructed and will probably be ~1MB, whereas the comparable standalone rust compiler rustc executable is likely to clock in at 20MB. And you know what? That makes no difference in today's world.
Edit: made it explicit that I am talking about the compiler itself (which was the subject of the parent post I was replying to).
I estimated that the rustc will be 20MB based on my experience with linking LLVM statically in the past - I have not tried it recently, nor have I built rustc. I might be way off. Kibwen says his rust compiler clocks at 8MB - so it appears I am way off (or maybe comparing an unstripped executable to a stripped one)
To be clear to child comment: I did NOT intend to claim that writing something in C will provide a 95% reduction of code size.
EDIT: Servo, which is the largest Rust program in existence at the moment, is 21MB. Personally I doubt the above claim that writing this in C would reduce the binary size by 95%. I obviously don't have any statically-linked written-in-C web browsers on hand, but if I poke around in Chrome's install directory I see 85MB of DLLs (the entire directory is about 475MB, though surely not all of that is due to binaries).
Is the 8MB rust compiler statically or dynamically linked to LLVM? It's been a few years, but I never got a statically linked LLVM-using program below 30MB (unstripped, I think stripped was 16MB but I don't remember)