I played them both (although it was quite a long time ago for Legendary Heroes), and I liked them both, but none of them was able to suck me in for long.
In Legendary Heroes :
- The AI is much better.
- The tactical battles are more fleshed out.
- The monsters you encouter early on are very interesting from a tactical standpoint, they may require you to design an army especially to take them out (I remember the banshee's immunity to physical, or the bear's maul).
In Endless Legend :
- The game is much prettier.
- The factions feel very different from one another.
- There are more quests, and they are quite well fleshed out.
In both games, the early game (exploration, quests, monster fights) is very nice. But the late game (contest/war with other empires) is meh : in LH it is because factions seem bland and spells are unbalanced, in EL it is because AIs act dumbly.