Since both are physical equals, this all comes down to abilities and intelligence, which is where Slappy takes this. For one, Slappy has legitamite teleportation, while Chucky only has the very iffy slashportation. Not only that, but Slappy can teleport a mass amount of objects, shown when he stole all of Stine's books. Say goodbye to any of Chucky's weapons...
Not just that, but in general, Slappy has shown plenty of abilities that would give him an edge against ol Chucky. Like size manipulation and mind reading. So this means none of Chucky's master plans will end up working here, since Slappy will always be able to stay two steps ahead.
This leads to intelligence, and while Slappy is indeed arogant, he's still smarter. He schemes more often than Chucky, shown when he often manipulates people and monsters alike, moreso than Chucky who prefers brandishing a knife and stabbing his victims. I'm not saying Chucky can't be smart, I'm just saying Slappy relies on tricks more, and is therefore better at pulling them off.
Final point, while neither seem to die for good, Chucky is always brought back by an outside force, while Slappy has been brought back multiple times in the book with no explanation as to how he returned from his destruction in the previous books. But that doesn't matter as much as my other points, it's just something I wanted to point out.
So yeah, Slappy should win here, The new film ain't gonna help things either, since it appears as if Slappy will be able to turn mere Halloween toys and masks into large creatures...