The wiki can help with that... in bits and pieces.http://www.exiledkingdoms.com/wiki/
The biggest advantage (rather only) is the access to the Golden Hand trader, that sells a few nifty items early in the game. But, with careful quest planning, one can access the library and the library's shops in one sweep and than not worry about irking the Golden Hand.
There's no real advantage to helping the Varannari, other than it's the decent thing to do. Some quests that help the Varannari give items. One can reduce negative Varannari reputation but not hunting escape slaves or hunting down Varannari witches.