In some patients the teeth are so crowded and the jaw so small, among other factors, that healthy premolars have to be extracted. In other patients, moving the side teeth back to create space is entirely possible. The patient's diagnosis and needs point to which method is more suitable. If, after the orthodontic analysis, you face a choice between these two treatment options, you should weigh up the pros and cons – treatment time, the cost of the appliances, the expected final result (are there any differences?), and how stable the result will be over time.
This answer is for general information. Your own case needs an examination by an orthodontist.