I'm gonna go with Archer. While Sayaka has the speed advantage, and her healing factor would make her really hard to take out, Archer's near-infinite storage of weapons would give him an upper hand. Each time he summons a weapon, Archer becomes just as skilled with it as it's original wielder, giving him years of skill for a weapon he might not of even touched before. With each new fighting style Archer takes on, the more time Sayaka would need to adapt to it, and the more chances she has to take hits, and if Archer summons Unlimited Blade Works, I don't know if Sayaka could handle all the weapons flying at her. If Sayaka transforms into her witch form, that would make her bigger, and possibly strong enough to take Archer down, but considering how just one arrow from Archer could destroy a cemetary, and how his Excalibur Image has enough power to wipe out entire armies, I think Archer has enough firepower to kill even Oktavia.
I gotta say Archer would take this. He appears to have the edge in strength and his feats are a lot more impressive. And like Digger said, he definetly has enough firepower to overpower her healing factor and witch form.