You can use either, but since GitHub allows us to more easily track the state of bugs and attach source code fixes, it is more efficient for us to process bug reports and issues via GitHub.
That said, if you feel more at home with JIRA, feel free to use it.
Only one request in that case: To cut down on us having to clean up duplicates, please only report a bug in EITHER GitHub OR Jira. Only when you wish to add a source code submission after you've already used JIRA should you switch from it to GitHub. In that case, just leave a link at JIRA and close the issue (or ask for it to be closed).