I think Armies of the Abyss is a great complement to Legions of Hell but you can't expect it to be exactly the same as the focus is somewhat different.
LoH is, with the exception of a few pages at the beginning and end of the book, a monster catalogue. True, the monsters have a lot of plot hooks inherent in them, but that's really all it is. AotA is a more ambitious product in a lot of ways: it features a new way of casting divine spells (sorta, the differences are more roleplaying rather than mechanical), new domains, a new core class that gets divine spells from calling on demon princes and has a lot more world-building stuff. What were they calling it in the other thread? Flunch? Adaptable fluff? Because of this, even though the book is bigger, it has less creatures in it.
However, the creatures it does have are just as interesting, and the "adaptable fluff" is so chock full of potential plot hooks that you almost can't touch it without getting your fingers snagged.
I think I've reviewed both of them on the reviews page, actually. I know Psion and a few others have reviewed both, so you can also see how they compared the two to each other.