The Campaign from Fallen Enchantress is being added to Legendary Heroes in the upcoming 1.4 update.
As for the sandbox mode, there are several different elements to the play style. Examples are:
- Heroes are attained through Fame (a new resource) instead of found scattered around the map
- Each weapon type has a different special attack
- Encumbrance has been stripped away from armor
- Influence has been stripped away.
There are other changes as well. Overall, imo, once you switch over to Legendary Heroes, you'll never want to switch back....but I have read on these threads of one or two people doing just that....so it's ultimately up to you. I very much like Legendary Heroes more than Fallen Enchantress. It's a more complete, more full, game to which the developers are placing more effort into making even MORE out of with the upcoming 1.4 Diplomacy update and the 1.5 Wrap-up update. DLC content is also being made for Legendary Heroes...don't know if it's backward compatablle with Fallen Enchantress....I doubt it.
Anyways, I suppose to summarize, almost everyone is onboard with Legendary Heroes being 'the game'.