They are more or less the same, though depending toning balls can bounce, or have a special grip. Not all, but sometimes that's how the distiction is set. If your talking the Danskin toning balls you see at Wal-Mart, they are the same as a medicine ball, and don't bounce.
What to look for:
1. Easy to hold, lift. Don't slide and are soft enough that you can get a good firm grip.
All other features will depend on what your going to do with it. If your thinking Cathe, about any of them will work. I personally like the sand based ones the best, they give you the best grip, as all you do is squeeze and it forms to your hand, or squishes between your knees or where ever you use it.
But if you want to be able to toss the ball back and forth between two or more people, get one that bounces, otherwise if you mis, your ball can go splat instead. Then you get to buy a new one.
Start with a light weight one first, the exercises are a bit different, and less weight is easier to control then heavier weight, use it and get use to it. As well as to understand form, and really see where that weight works for you best. Build up your muscle and then it's the same as dumbbells go up to the weight that's right for your level.
Kit