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'.

AFTER reading your answers i think i ll wait to 1.4 update because i love to spend much time in comagins.

I've only played WOM and FE campaigns. The campaign is a scripted RPG mode. It's designed to tell a story instead of teaching you the game like some other games. I will point out if you hate reading in games don't bother. Without reading the text the campaign will likely not be very fun. 

Until the later stages, then it gets fun since you've spent time investing in research etc. and hopefully are fighting hard enemies. A good campaign has a training mission (skippable), and an increasing difficult progression throughout. (I only like the campaigns in HOMM4 for instance, the scenarios were pure crap, and HOMM3 was better for that replayability, and still is... )

I like games with story so always i like comagins  , second thing i play presence of a very bad Internet connection in my town so off-line content of any game is very important to me.