In my experience, having done both, it is a bit more time efficient at the Costco, but it also changes what you buy (lots of fresh food doesn't actually do well for two weeks). This is assuming you have the places you need in easy walking distance of course. At one time I had a grocer/butcher/wine shop/etc. all literally on the way I would walk home from subway anyhow, so my overall time was probably even shorter than a big grocery run every 2 weeks. I paid a bit more, but then I got better produce and bread, and could change meal planning on a dime if I saw something good. All tradeoffs I guess.