Free weights and machine weights
Safety and being injury free should always take precedence. If you cannot safely perform an exercise with free weights safely, yet can perform a machine version of it always go with the machine.
However, if you are able to safely and productively perform a free weights version of the exercise (e.g the squat) always go down the free weight route. Free weights involve far more musculature than machine exercises due to the stabilization muscles being involved to a much greater percentage.
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Free weights can also prevent injury because they allow for a far greater and natural movement for the individual, however, good form is paramount! Machines are much more restrictive as they tend to isolate muscles but they can also hurt users over time.
Generally speaking, free weights are better tools for the job, they are also much cheaper and versatile.