What is different between regular Catan and Simply Catan?
It looks like the biggest differences are tile sets (multiple hexes attached together) to decrease set up time, and only 7VP needed to win for shorter games.
From the Catan website:

Differences Between Simply Catan and the Base Game
For a quick setup, Simply Catan comes with 5 rows of tiles, printed on both sides (1x5, 2x4, 2x3). This design reduces overall setup time of the game while still providing enough variation to make for a different game each time you play.
For beginners, the rules provide recommended starting positions. This allows new players to dive immediately into the fun of the game mechanics, without spending a lot of time worrying about getting started.
Playing to 7 Victory Points speeds up the time to play. With Simply Catan, 30-40 minutes is all your family needs to get your &quot;Catan fix&quot;.