You really picked a hard question Arcademan/PixieP-san. It had me really got me thinking for quite a while, but I was able to come to a decision. I'll first give the pro's and con's of both Syaoran's and then tell you who I voted for.
CCS Syaoran Li Pro's: This Syaoran would be the obvious choice, for a number of reasons. For one, he knows martial arts, can fight with a sword, has the ability to use magic and when fighting, and he doesn't give up.
CCS Syaoran Li Con's: While he still has the ability to do all of the above, he lacks one ability. When Syaoran Li fights, he does it to the best of his ability but when Sakura is involved, he lets his emotions get in the way of his fighting. He often becomes jealous, protective, and nervous, and the fact that he is 11-12 years old, he doesn't completely understand his feelings, and this is his downfall.
Tsubasa Syaoran Pro's: This Syaoran also has a number of abilities. He is also able to do martial arts and he also is able to overcome a fighter with magic.
Tsubasa Syaoran Con's: The only weakness I can see is that he is blind in his right eye, but at the same time it is an asset. If Syaoran wasn't blind, Seishiro-san would have never taught him how to fight.
Those are my reasons and now I'll give my answer. If you asked me which Syaoran was my favorite from each show I would definately choose CCS Syaoran, but I won't explain why because that's off topic, but the Syaoran that would win the fight would be Tsubasa Syaoran. For one reason, even though he cannot use magic, he can win a fight against a person with magic. He has proven this many times, but the main reason would be that CCS Syaoran does let his feelings get in the way of battle. His emotions hinder him, and cause him to lose a number of times and it's not entirely a bad thing. For one, as stated, he is only 11-12 years old and is just starting to learn what real love is, while Tsubasa Syaoran, no matter what his emotions are, he doesn't let them get in the way of his fighting. It was a very tough question and I commend you for it Arcademan/PixieP-san. For giving me such a brain teaser, I'll give you a cookie!