More repetitions or more weight?
In general, exercises with more repetitions are performed for the improvement of muscular endurance, while those with a higher weight and fewer repetitions are practiced to increase muscle size and strength. Here are some guidelines to start. You may want to work with a personal trainer or other professional to develop a program that allows you to achieve your goals. To lose weight: from 1 to 3 sets with a load that allows you to perform 10 to 12 repetitions with effort.
To gain muscle: 3 or more sets with a load that allows you to perform 6 to 8 repetitions before you tire. Beginners should start working at this level.
To develop resistance and physical fitness in general: 1 to 3 sets with a load that allows you to perform 12 to 16 repetitions with effort.
Another question that we frequently ask ourselves is whether we should rest between the different series. The more intense the training, the longer the breaks will be.
If you work with heavy loads, you should rest a time between 2 to 5 minutes after each series.
Note for women:
Many of you miss the benefits of bone growth that strength training provides for fear of "widening." Do not worry. The hormonal composition of most women will prevent their muscle volume from drastically growing.
Text taken from: http://www.precor.com/es-co/%C2%BFm%C3%A1s-repeticiones-o-m%C3%A1s-peso-aprenda-focusing-your-training