Neither of them are particularly fast, I agree, but Antony has shown better endurance and more consistent defensive work for the full 90' on the wing. Sancho, until his comeback at least, was fading around the 60' mark and wasn't even a high energy player during those 60 mins. His defensive work was a bit shoddy. Overall, Antony was more tactically aware: hugging the line and stretching the pitch to create space, while also doing a lot of pressing, running and covering of his fullback. These are things that ETH expects of his wingers.
I have to say Sancho has improved his endurance and work rate since his come back, particularly evident last week in Barcelona, but he's not yet done it anywhere near enough or as consistently as Antony. The latter is still the better winger. But Sancho's ability to find passes in tight spaces, play one-twos and score goals from inside in the box is clearly superior to Antony. Which is why many believe (me included) that a central or free role suits him better.