Biggest difference is indeed playoffs. Thornton would very likely have managed that feat if league wide scoring had been anywhere near the level it has been lately. He generally stayed healthy and was a strong point producer until he was in his 30s.
I don't really think there is a case for Thornton given the difference in playoff performances. I do think they're very comparable regular season players. Thornton never really had a very good playoff run. Draisaitl has several excellent ones.
I think regular season production in his prime is still firmly in Draisaitl’s favour relative to the league. They have similar top 3 years or so but Draisaitl’s 7 year stretch here is already better than Thornton’s best any 7 years let alone a consistent stretch of play, and he already had more top 10 finishes. And while Big Joe aged fairly well, he only had 3 years after 30 in the top 20 in league scoring. I think even as a regular season player, Draisaitl will end up on a level between a McDavid and a Thornton.