I guess it depends a little bit on how the github is currently used and its audience, compared to this Discourse, when it comes to discussions. To me issues on github (in general, not just with Julia) usually feel more developer-oriented and less user-oriented. A post here will also reach a wider audience I suspect, as not everyone closely follows activity on github. So I assume you will get different feedback and discussions depending on the platform chosen. Also, I personally find Discourse to be nicer for holding discussions, due to easier quoting and searching.
Python seems to have chosen for their Discourse forum to discuss PEPs (PEPs - Discussions on Python.org), by the way. But that might have something to do with them still using their own issue system instead of github.