Engine oil is a complicated topic and there will be differences between the two, down to a combination of chemistry and marketing. But with your usage pattern, the differences between the brands do not justify the difference in price. If you want to save money, keep an eye out on the supercheap and repco catalogue. Several times a year, they have 5w30 or 5w40 on specials down to around $30 - $35 for 5L.