If your intention is to upload to those sort of portals it really mostly comes down to your personal preference; which are you more comfortable with?

Is there anything particular about your game that might make it more or less well suited to either option?  (Example: non-trivial audio can be tricky in HTML5 (although there are solutions available)).

What’s your plan (if any) to monetize?  Some advertisers only support Flash or only support HTML5, sponsorships can be less common for HTML5, etc.

Flash is becoming less common on mobile devices if you want to support those.