costco has better meats and in my opinion nicer merchandise throughout the store... but sams will have better supplies for the caterer
But, costco here only carries boneless boston butts, but their ribs are nicer than sams. But Sams carries alot of the kind of local wines from local wineries. Costco's cakes and bakery are better than sams....
go into both stores and check them out... see what you like best... they will let you browse before joining.
ohhhhhhhhhh... and at Costco, I have the Executive membership which costs 100 bucks a year, but you get 1 or 2% back.... more if you get the american express costco rewards card.... this alone more than pays for the $100 membership fee......don't know if they have that at sams.