Unless you have an injury free weights should always be your choice of exercise for several reasons.
Here are a few advantages of free weights:
- Using barbells & dumbbells are much more effective in strengthing/developing synergistic muscles, e.g. rotator cuff.
- Biomechanically free weights match the natural movements of the body.
- Overall you will develop greater strength & size using free weights.
- Overall power is achieved much more efficiently.
Here are a few disadvantages of machines:
- Machines force the body to maintain a pre-determined & often unnatural path.
- Machines are not very effective in strengthing/developing synergistic muscles.
- Machines are built to accomodate the "average" person's body, so if you're larger or smaller than average you may not fit very well.
- Difficult to achieve maximum velocity due to the inherent constraints of a machine.
Now do not get me wrong here, I am not stating that machines should never be used. Machines do have their uses, but the majority of your exercises should be done with free weights. Machines do have some advantages as well.
Advantages of machines:
- Some machines can be more efficient in muscle isolation.
- In some respects, especially for a beginner machines can be safer.
- A quicker work-out can be done utlizing machines, no plate changing, etc.