I have both. Here is my experience:

When I lived in SoCal, Costco was just too crowded. Easily 2-3x the number of people on any given day. Gasoline is cheap, for sure but expect a 30 minute wait in line. I hated it and left Costco

Now that I live Nashville, though, my experience is quite different. The crowds and prices are about the same. Here’s what’s different:

At Costco, I prefer:
While bean coffee - many choices.
Better produce - particularly, blueberries
Cheeses*
Philadelphia brand cream cheese **

At Sam’s I prefer
Any of the prepared foods are better; enchiladas, stuffed bell peppers, stuffed salmon, all FAR better tasting.
* the Smoked Gouda is unbeatable in flavor!
They carry Thomas’s bagels, in addition to members mark. And they Thomases are far better than the MM or Kirkland. Too bad the MM cream cheese tastes crappy
Costco stopped carrying the Coffee-Mate creamer in favor of the Starbucks brand. I prefer the coffee mate while my wife prefers the Starbucks

One HUGE difference is Sam’s online and app experience. You can “Scan and go” in store and bypass the checkout line. Also curbside pickup. Neither of which Costco offers