Most make the mistake of using a ball thats too heavy. We rarely go over 8lbs with MLB players, so I usually like to stick with 2-6 lbs for youth.
The key is developing usable power and proper movement techniques. Using a heavier ball will not make you "stronger" if it causes you to lose posture and technique.
Typically if you're throwing the ball against the wall, you'll want one that bounces. And if you bounce on the floor, at times you'll want one that bounces and others you'll want one that doesn't bounce (unless you want to get it right back in the face).
I'd get 1 of each if possible.