Legendary Heroes is an improvement over the other game. You would be missing on the campaign from the previous game, also different mechnics regarding heroes and champion level ups, but in essence you are receiving the same game by purchasing Legendary Heroes. Also, if you read the forums, you will find a vareity posts of saying that this is a far better version than FE, and I tend to agree with them.

Yeah, don't feel bad about skipping Fallen Enchantress your first time around.  Scenarios aside, both games are very much stand alone, and LH has a lot of cool stuff in it.  Legendary Heroes looks to be quite the fun game for the first timer getting their feet wet in the Elemental setting.  Plus, some nagging bugs that lingered in Fallen Enchantress have been dealt with in Legendary Heroes, so it seems more stable to me, which is a good thing.

You can always go back and snag Fallen Enchantress later when it hits the bargain bin or whatever.  Or, if you REALLY like LH, you could grab it sooner if you can't wait for the deals to surface...