HAHAHAH! (I'm a sadist. I find laughter where ever a slightly one-sided heated debate takes place.)
The reason why I think Sasori actually lost to Sakura and Chiyo is mainly because Chiyo was a puppet user, making it the reason WHY she knew how to control Sakura for her to dodge all those needles. Moreover, Chiyo was the one who taught Sasori how to use puppets so she should have an idea of his style. Kakashi, with his sharingan adding precision into his nimble abilities, could most likely manage to dodge the needles. But, no matter how much I love him and I REALLY do, there're just so many needles that he's bound to get scratched. And the time when he'll get scratched is when the battle drags out for too long, thus making him slow down.
Kamui, yes it's a strong move, I would TOTALLY know... (I'm not trolling on Kakashi. I love him very much, but I'm trying to look at this at an unbiased view.) But Kakashi starts faltering after one use. And as for the range of what he can suck into it, I don't think it can suck all of Sasori's puppets. Of course, he could always aim it directly at Sasori, but Sasori's not just gonna be a sitting target like Deidara.
Hey, I love both characters, and I hate being wishy-washy, but I honestly would have to SEE the fight to believe this particular one. Sasori's strong, but Kakashi's craftier.
I would love to see this happen instead of any of the other battles that have been going on in the manga. (Sheesh, I really don't care about Darui since he's like a filler character to me and no one wants to see Choji turn into a freakin butterfly...)
So, I basically just named the faults of Kakashi. Who wants me to name the faults for Sasori? lol