How about another Who Would Win? Let's go with Harry Potter vs. Harry Dresden.
I think Dresden has the upper hand in a fight. He's older, smarter, more experienced, and more powerful. He should be able to kick Potter's tuckus halfway to next Tuesday. On the other hand, Potter is just a kid--Dresden probably holds back. But on the other other hand, Potter would be holding back too--he wouldn't be willing to fight dirty. Dresden is more pragmatic. He'd be throwing spells, but he'd be throwing punches too. And even if Dresden hesitates, Potter isn't skilled enough to capitalize on a brief opening like that.
Yeah, I guess this one's too easy...they're just in different weight classes. Dresden vs. Dumbledore, that's a better matchup, I guess. On the one hand, Dumbledore is the most powerful wizard in Harry Potter canon. On the other hand, wizards have a higher power level in The Dresden Files canon, and Dresden is definitely above average. Eh, Dumbledore is pretty badass when he cuts loose, though. I'd put the odds somewhere around 2:1 in Dumbledore's favor. Dresden is likely outclassed, but he's good enough to put up a decent fight and capitalize on any slip-ups. But I'll freely admit that I might be misjudging Dumbledore's power level--he doesn't show off much in canon, so it's hard to say. I do think the odds are definitely in Dresden's favor against any of the other Harry Potter wizards.
Well, there's another Who Would Win. Always a good brain-stretching exercise.