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 tu talking about who has más 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 por quite a lot.
Conclusion~ It actually just depends on the individual's opinion.
Well, I think it depends on the conditions. If it is a close range battle o hand to hand combat then asuna is better but if it is a long range battle then sinon has a higher chance of winning.
Asuna because she was known as the lighting flash asuna which is a diffcult título 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 más 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