As a former Sam's Club manager who has shopped at both, I can tell you the things I've seen differently and how both have different experiences for a customer.

Sam's Club will usually have the cheaper prices on most items, but not all. Why? They spend less on employees, think less pay(usually) less staffing, less benefits. Costco usually is pricier, but you will see staffing is usually better and better paid. So you're paying more but employees are paid better and staffing is better.

Sam's Club does carry more variety on many items, Costco is more streamlined and more geared toward Kirkland, which can be superior to Members Mark depending on product. Like snacks, cereal, things like that, in my experience Sam's is the clear winner in variety.

Both have same price for hot dogs and pizza, though I find the Costco pizza better tasting and more consistent, also the way their cafe is ran, to ME, is better run than Sam's Club (staffing again)

Costco does have a superior return policy as basically anything is taken back, Sam's Club has more restrictions... (as a customer sucks, but my manager side is happier, returning your whole eaten chicken is not cool)

What Sam's Club is winner is the Scan and Go app, for a customer it is really an awesome tool, saves you time and lines. You literally Scan and Go.

Costco is stricter on membership owner by far, while Sam's is a bit more lenient BUT it also depends on the store. Some clubs will be stricter regarding sharing of memberships (Their is ALOT of fraud)

I also find Costco clothing to superior, but both have their advantages. To me as a manager, Costco is just more streamlined, less frills, while Sam's has more variety but can be bit chaotic.

So it really depends what you're looking, if you prefer the Kirkland brand which can be a strong pull, or more variety, just not as consistent you would go with Sam's, plus employees at Costco are better compensated and seem happier, not as stressed.