The Philips wrist blood pressure monitor can be used on both arms. Most people prefer to use it on the wrist of their non-dominant hand. (For example: on the left wrist if they are right-handed and vice versa.)
For a meaningful comparison, try to measure your blood pressure under similar conditions. For example, take daily measurements at approximately the same time. Try to measure on the same arm, or as directed by your doctor.
If your doctor has diagnosed you with poor circulation in your left arm, use your right arm, and vice versa.