We buy both but I think we do purchase more store brands overall. One of my stores has started carrying store brand organic items and I feel they offer the same quality at a lower price, especially the whole wheat pasta.
DH refuses to use anything but JIF peanut butter and actual Miracle Whip for his sandwiches. No store brand for him on those 2 items! I've tried store brand cream of mushroom soup and have been disappointed so now just stock up on Campbell's when on sale or have coupons. I make a lot of my own salad dressings but when I buy dressings, I have some favorite brands.
Some suprising store brands that I really enjoy: Market pantry (Target brand) ground coffees and ultra pasturized creamer; Walgreen store brand medications (equivelant to tylenol, benedryl, sudafed, children's tylenol, etc), toothpaste, body soap (like Dove sensitive skin), hand sanitizer; Sam's Club brand diapers, pull-ups and hypo allergenic baby wipes; Wal-Mart diapers and pull-ups (we don't need any of those now but will in the next 5-6 months!).
Some brands I am partial to: Aveeno facial cleanser and baby soap and lotion (both ds and I have sensitive skin and Aveeno works best us); laundry detergent (either Era or Purex free and clear); Modern Organic Product shampoo (expensive but my head does not itch using this shampoo) and Lubriderm and Eucerin lotions.
These I look for on sale or coupons. I do try the store brands and if they work, I do switch. Sometimes with a coupon, the regular brand is cheaper than store brand so I keep that in mind when shopping.