by LeeB1985
As people have said they're both compatible both contain 6 different episodes, characters to play as, a set of insanities and 2 bosses. The only think you'll double up on from having both is the basic components (dice, tokens etc).
As others have said the original core (or season 1) is tried and tested (and very much loved). It's a great starting point and most people who like this game tend to pick up more content.
Fear of the unknown (season 3) does add an extra couple of modules that can mike it more difficult/easier and add variety so could also be a good place to start especially if you don't want to expand past one box.
Personally I'm looking forward to season 3 as an expansion to a game I already own and love.