Strength~ Well, we haven't seen them fight each other but based on their weapon choice and the battle with Zekken (Yuuki), Asuna has a better chance of being stronger.
Fans~ If আপনি talking about who has আরো fans, then based on MAL, it seems people seem to side with Sinon when it comes to polling. Though they do, when it comes to favorites, Asuna wins দ্বারা quite a lot.
Conclusion~ It actually just depends on the individual's opinion.
Asuna because she was known as the lighting flash asuna which is a diffcult শিরোনাম to claim while Sinon is a just getting started to ALO and stuff.In GGO sinon isn't as powerful.She got shot with a silent bullet I think and was about to get killed.If she stayed on guard she could of noticed the bullet sooner.Asuna is sorta আরো powerful that Kirito when she fought Yuki in ALO.Yuki accepted her into her party.While she could of gotten kirito.So that is why I think Asuna Yukki is better than Sinon