This is one of the smaller CostCo I have been to in GTA. The selection is a bit odd as in my experience other than certain region/demographic items which are location specific they pretty much carry the same variety at most locations. Surprisingly this location is a bit lacking when it comes to cheese/produce as I missed out on the certain staples. The setup is like most other stores and on a weekday evening there was a spartan crowd which is appreciated.
There were no items that stood out for me. So while comparing it to other CostCo's this location is 3.5 stars (still 4 stars over other grocery stores)
I haven't been to the Markham location so can't comment which one is better.