are movable wingtips better than canards?
Two different designs. Delta canards need canards in order to bring the center of lift forward, because their wings are so far back. Movable wingtips are no different than elevons and ailerons. They just pivot about their center instead of hinging at an extremity. This makes them larger and more efficient at redirecting air.
If we're talking stealth, then a movable wingtip would be better by virtue of being part of the wing and not an entirely new surface.
The speculation is that they're a replacement for vertical stabilzers
Highly doubt that. If the wingtips deflect one way and ailerons the other in order to generate drag and achieve yaw, then one might as well go ahead and use split rudders instead of subjecting the control surfaces and wing to all these unnecessary bending moments and torsion. The J-50 already looks stable enough, directional wise. It has a void on its belly starting from between the intakes and all the way out the back between the nozzles, and it has another one on the top right behind the cockpit and out the back between the nozzles, somewhat like a YF-23. It will likely yaw using some sort of thrust augmentation/bleed, or split rudders, or both. We won't know until the design is finalized.