For the average infrequent traveller, I don't think you would notice any difference. They are pretty interchangable.

For the business traveller, there's a vast difference. WS's frequent flyer program is awful, and if you aren't YYC based and can take advantage of the Elevation lounge, provides nothing for higher end flyers. You don't even get seat selection on all flights, just a few vouchers. On AC, I get preferred seat selection on any purchase of Flex or higher.

The upgrade system is a joke, as mentioned above, they are giving seats away to any idiot for nothing. There is zero that is exclusive and elevated about Premium, either in the offering or the people flying it.