1. Delera has better questdesign, better loot, better levelrange and it is generally much more fun. Also, It is narated by Gary Gygax, which is pure win.
2a. First quest miniboss is commenting on what happened in the Harbringer storyline. Aside from that, these two dont really share any story.
2b. I THINK that Harbriger drops some loot, that can be upgraded in Reign. But if so, (I am not certain), you can definitely live without it.
2c. Better loot comes from harbringer, reign has decent scimitar, crappy staff, mediocore armors that can be upgraded (which does makes them interesting and that is pretty much it. Harbringer /first pack/ has second most powerfull heroic Bow, awesome quiver, DR breaking rapiers, very good greatsword and very decent light armor parasitic breatsplate. It also has somehow usefull bracelet and crappy dagger. Ah, and he robe from there is liked by some.