Emacs' Evil Mode is on-topic on both sites, but the scopes are slightly different.
Derobert's Vi and Vim Meta answer to a question linked from the Vi and Vim Stack Exchange help center states (emphasis in original),
- It need not be a stand-alone program; a plugin for Visual Studio that gives vi-like behavior is on-topic, but only the vi-like behavior is.
On the other hand, Evil Mode is part of Emacs, and so is on-topic at Emacs Stack Exchange, where there is currently an evil tag with 424 questions.
The best site would, therefore, depend on the kind of question you want to ask about Evil Mode.
- Questions about the vi-like behavior of Emacs' Evil Mode would best belong on Vi and Vim.SE, as they are likely things that tend to be important or relevant to a vi enthusiast.
- Questions that are more or less Emacs support questions that somehow involve Evil Mode, like how to install Evil Mode, how to switch to it, or why you get an Error 553 when you try to use it in combination with the Klingon script pack you found on some FTP site would best be asked on Emacs Stack Exchange, because those are the kinds of things that an Emacs expert would likely understand or care about.
For your example question, it looks like a question about the vi-like behavior of Emacs Evil Mode rather than an Evil Mode-related Emacs support question, so Vi and Vim SE would likely be the best site.