One form (saw), is in the simple past tense and the other (have seen), is in the present perfect. We use the present perfect when talking about actions that continue to be true in the present, or in other words, the time period has not finished. Also, we use the present perfect when talking about subjects when the time of the action is not important or not specified. Look at the following example: 'I saw 'Star Wars' in 1982' The action started and ended in the past. versus 'I have seen Star Wars 17 times.' It doesn't matter when you first or last saw it, you are indicating that there might be a chance you will see it again which means it is an action which began in the past and continues in the present. 'I saw John three times last week.' That time period has ended and therefore in the past. 'I have seen John three times this week.' The week is not over, therefore this action continues in the present. 'I saw that exact woman here last week on Friday.' Ended in the past on a specified day. 'I've seen that woman before.' No specifics as to when in the past but the action continues in the present. Hope this helps.