M20 is more RPG light. In this genre I prefer Dungeonslayers (D20 is not my cup of tea)
Unfortunately Advanced HeroQuest is not produced anymore. And I do not have a copy
(and I do not want to spend a lot of money on ebay)
Song of Gold and Darkness is a very nice supplement (i´m a big Song of Blades and Heroes Fan)- With Song of Gold and Darkness you can play a Temple or Castle raid but no classic Dungencrawl (only my opinion)