S-6 is part of the spec the wire is manufactured to meet so I doubt you would be able to notice much if any difference between brands. And like everything these days, one factory probably makes a majority of the wire and brands it according to what the customer asks for.
Cheap is a relative term. 2#s of wire for $13 is $6.50/lb which is high to me considering I pay $70 for a 33# spool.
Bottom line, if it works for you and your purpose, does it really matter the label or other people's opinion?
I buy from my local welding shop,