Both help; what matters most is using enough and reapplying.
SPF 30 filters about 97% of UVB rays and SPF 50 about 98% when used as directed. Pick SPF 30 for everyday errands and SPF 50 for beach days, sport and long hours outdoors. Either one only works if you apply enough, about 30 ml for the whole body, and reapply at least every two hours.
