손님 can be a guest or customer, like at a hotel they would call you that. Some places call you 고객님. 아저씨 is very general, you can use it when getting someone's attention on the street for example.
손님 is also a general mister whereas 아저씨 usually refers to someone middle aged. You could call a man in his 20s 손님 but not 아저씨