hard to say really.
strange had all those books and those books have a shitload of pages, and magic breaks universal rules (or just ignores them), whereas the power cosmic can pretty much replicate anything that occurs naturally in the universe.
they both have had battles with beyond sky-father level beings, but being the sourcerer supreme is pretty much your duty to fight and keep beings like that out of the 616, and he's done better.
Yeah, Classic Strange was on a different plane. Even the normally un-trump-able Surfer has to take a back seat, in both versatility and total power. For example: Can Surfer defend against a skilled wielder of each one of the Infinity Gems, then the IG as a whole? Hell no. Strange could. Doesn't make him equal to teh IG (obviously) but he's just in a different conversation at that point.
Non-Sorcerer Supreme Strange loses, though, which should be just as obvious.