I played XI recently.
What does S offer over the original, besides 2D and orchestra?
list of the new features coming to the game in this version:
- "super fast" battle speed
- japanese VA
- party members follow you on the field
- you can call your horse from anywhere on the field
- killing enemies while on the horse earns you XP
- the 2D mode from the 3DS version (you can play the whole game like a SNES RPG, and can toggle between 2D and 3D mid-game)
- orchestrated music, can switch back to MIDI (it's still not clear whether everything is orchestrated or not, but field and battle themes are confirmed)
- new story content, focusing on each of the party members
- more monsters to ride
- the retro areas from the 3DS version are back, you get to revisit areas from each of the past 10 dragon quest games
- more marriage options (you can choose to "live with" male characters as well)
- there's going to be an audio drama DLC coming after launch