I'd use both, rotating between weight belt and bodyweight. IMO, pull-ups are one of the best bang-for-the-buck exercises, which is why some call it the 'upper body squat'. I do a couple of weeks without a weight belt and try to up my total reps per set (e.g. 15, 12, 10, 8). Another week, I'll add a weight belt with a plate and bring the rep scheme down (e.g. 8, 6, 5, 5), increasing the weight as I'm able.
Also, change your grip to target different muscles (palms facing you, palms away from you) and change the grip width (wide grip pull-ups vs narrow grip). If you want additional benefit for your abs, do pull ups with your legs straight in front of you, with your body in an 'L' shape, like a gymnast. There are so many variations, it's easy to make pull-ups the cornerstone of your upper body development.
