Should I buy more expensive oats?
My experience is that for making porridge (various methods, sometimes soaked but not always) or granola, or for baking (flapjack/oat bars, crumble topping), all rolled oats are equivalent. I usually buy the cheapest (bottom shelf, boring packaging in UK supermarkets) but occasionally have to get more expensive ones.

One thing occurred to me that might possibly make a difference: some, such as Lidl's cheapest that I currently have, are packed in a paper bag. Many others, including the more expensive ones I've bought, have a plastic bag, possibly inside a box. If you're storing them for long periods in a humid place or exposed to strong odours, the paper may not be as good. Of course you could always put them in a sealed container when you get them home.