If the tackled player is isolated from his support then go for the ball. If there are attacking support players arriving, beat them to the ball by driving past the ball/tackled player so you are between them and the ball, your supporting players bind on to you, you effectively make contact with the opposition with neither of you touching the ball but forming the 'perfect' scenario of a ruck, players on their feet trying to drive over the ball while the tackler and tackled player are still on the ground with the ball. Practice by setting up the different scenarios that make you choose the right decision, slowly increase the pressure on yourself by speeding up the scenarios so your reaction time is improved.