I did not get into details in regards to CO2 but, from my diving days, I've fully researched other gases and the only difference in between the different grades of gas is the filling procedure. For welding they do not vacuum the tanks, just "top them off" to spec pressure, for "diving grade" (places that carry it, not too many) they will fully vent the tank and some will even vacuum it, for lab grade the tank itself gets inspected for foreign matter, vacuumed and after that filled, all the gas comes from the same liquid storage tank. If you own your own tank and the facility fills them straight from the liquid storage I would get the cheapest grade they have.