I bought the individual seasons too.
My Season 1, 2, 3 & 6 are double sides discs. Seasons 4, 5, & 7 are on single sides discs. I wish they were all on single sided discs. Sometimes double sided discs can have problems.
I would buy whichever set is on single sided discs.
As far as I know it was never remastered in 16:9. They just keep putting the series out with the same transfers only in new packaging and the later discs being single sided.