I got another great question from Channa today that I thought I'd share with all of you. You may have heard by now that the main city in the Outlands (Shattrath City) is neutral, but houses two conflicting factions. Players must choose a faction to affiliate with and gain access to banking facilities, etc. Once you become friends with one group - you will be an enemy of the other. These two groups are the Aldor and the Scryers. According to the WowWiki, the Aldor are an ancient order of Draenei priests and the Scryers are a group of Blood Elves who broke away from Prince Kael'thas. Both groups now compete to win favor with the Naaru who control Shattrath City.
It might seem obvious that the Aldor are Alliance-oriented and the Scryers are Horde. One of the beauties of this system is that players can opt to "cross party lines" and make friends with the opposing group. One of the main benefits of choosing a faction is to gain access to new recipes and gear. For the professions that make armor, the Scryers' recipes allow you to make gear with arcane resist. The recipes from the Aldor will allow you to craft fire resist gear, predominantly. There are some other items they sell as well, but minimally represented. Alchemists will probably want to go Scryer since the Aldor don't have any Alchie recipes.
Other than gaining access to specific types of resist gear, the choice is completely personal. There's no reason not to go with whichever group sells items or recipes you want to have for your character. Or specific items you believe will be strong sellers in general. Keep in mind that the Scryers will probably less popular with Alliance players, so if you go Scryer you may end up with recipes that have higher demand (or less competition with other crafters).
I'd recommend that you closely review the recipes and gear available from each quartermaster (located in the appropriate bank) and choose carefully before doing any quests in Shattrath City. Somehow I ended up with friendly Aldor faction and Hostile Scryer faction on Korlyn and Kaliope both without even officially choosing either side. I had done a few quests from the Lower City and apparently raised my Aldor faction without noticing it. So try to make a choice before you do any quests there or the choice may get made for you. Supposedly there's a way to win favor with the hostile group, but I assume it involves a lot of mindless grinding that most of us would rather not do.
Here's a nice page on the WowWiki that might help you review your options:
Comparison of Aldor and Scryer Rewards