Now for a more serious argument for Loki (gotta protect my boy)...
Strange has some access to powerful magics, but Loki's is more versatile because it is inherent within him. His Marvel bio says he's created dragons out of clouds...sounds to me like some magical matter manipulation. If that's true then creating duplicates, numerous distractions and alterations, or changing the environment of a fight to suit his needs is not out of the realm of possibility.
He's also given magical powers to people, can moonwalk between dimensions (mainly Asgard and our dimension), and has pretty much anything else that Strange might throw at him (telepathy, telekinesis, teleporting...running out of 't' powers, etc).
And being inherent within him, he will not only be more comfortable with his magic, but he'll be able to use it more quickly most of the time. In a long, drawn-out fight, I'd take Loki.
-DM