Conditioning gear does what dumbbells and barbells cannot: it lets you move fast, throw things, and finish a session without needing a spotter.
Smooth or tyre tread
Both shells hold the same sand fill and have the same dead bounce, so the ball stays where it lands instead of coming back at your face. The tyre-tread finish grips better once your hands are chalked or sweaty. The smooth finish is easier to wipe down and kinder to floors.
Which weights
Most people over-buy here. A 6kg and a 10kg cover the majority of throwing and slamming work. Go heavier only if you are doing carries and over-shoulder work, where 20 to 30kg starts to make sense.
Ball stands are worth buying at the same time. Slam balls left on the floor are a genuine trip hazard in a busy room.