Well if I'm debating the stronger from the two, I think it'd be Red because he's more consistent with his victories, became champion, and even defeated Mewtwo with Mega-Charizard to capture it.
As for who I prefer, well, I honestly like them both: Red is a trainer who stands up for what he believes in, even when it came down to stopping Team Rocket all by himself. He has his moments of development when Brock came to teach him about believing in his pokemon when he was struggling for the "answer for his travels".
Ash, while he's not the best trainer, he has a never-give up attitude that I admire, with a think-outside-the-box way of thinking in his battles. In all honesty, I think the majority of Ash's battles are more entertaining to watch than the few we see with Red - Red uses more power and takes advantage of type advantages during battle, while Ash sticks to his unorthodox methods. He does have great developments during X/Y, which heck, he's arguably going to his peak now!
So all in all, I like them both, but, I think Red just has the edge in strength, is all.