Similarities:
* Like you said, both have fewer choices than a conventional grocery store: if you want ketchup or peanut butter, there's only going to be one brand and one size.
* Both of them don't have scales at the registers: unlike at a conventional grocery store, nothing is sold by weight (which I'm sure provides another small efficiency gain).
* Both of them are cheaper than your typical grocery store.
Differences:
* I feel like Trader Joe's leans on store brand / white-labeling items more than Costco -- yes Kirkland Signature is a thing but Trader Joe's takes it further.
* The shopping experience is pretty different both in terms of the in-store experience and the quantities things are sold in.
* Costco requires a membership, Trader Joe's doesn't.
I wonder which elements of the two models would work best for a public grocery store.