I haven't experimented too much with the guilds, since I don't use magic (and you join a guild mainly for the new magic spells that become available). However, I have made some observations when I eventually joined all the guilds (using separate save files). Here's a quick rundown of the guilds:
Priestess: very little offensive magic, excellent restoring magic. Healing Cloud is the best - 300 HP heal vs. everybody for 120 mana! You'll also learn a nice spell that works great against the undead (*cough* Ahriman *cough*). The weapon you get, the Healing Staff, gives Talia a small HP heal every time she hits with it, but deals regular damage vs. monsters. You acquire the “Book of White Magic”, on which all the spells of a Priestess are written. Upon learning all the spells of a Priestess, the book isn't dropped, but remains permanently in your inventory. To join, go to Mysten Far and defeat the Priestess. When I did, my class boosted all the way up to the maximum of 75. I assume that this was a bug, since by the time you've reached class 75, you've got hundreds of thousands of experience points and no one monster can possibly give you that much!
Witch: excellent status-based offensive magic. The weapon, Thistle, looks pretty nice (a huge thorned vine), and it's pretty strong. You acquire the “Poisonwood Scroll”, on which the spells of a witch are written on. The Poisonwood Scroll doesn't get dropped when you learn all of the spells written on it, but remains as a permanent part of your inventory. Most of the spells are called “curses”, and one of them is costly, but drains a bit of HP & MP from your target, restoring Talia (though not enough to justify casting it). To join, go to Witchwood, and defeat the witch on the top floor of the tower (the one with the school in it).
Earth Mage: good all-around magic and cannot be defended against. The best (in my opinion) overall, with average qualities all around and no weakness. The weapon of an Earth Mage is the Earth Wand. You acquire the “Earth Spellbook”, on which the spells of an Earth Mage are written on. It doesn't get dropped when you learn all the spells, but becomes a permanent item in your inventory. To join, go to the Dwarf Mines and defeat the Earth mage (found in the small dwarven village).
Fire Lord: powerful fire-based magic, which works well on monsters that aren't fire-resistant, such as those in Tar Vendron Desert or the Fire Cave. The spells deal heavy-duty damage, and are among the strongest spells in the game. The weapon of a Fire Lord is the Fire Whip. You acquire the “Dragon Scroll”, on which the spells of the Fire Lord are written on. The Dragon Scroll doesn't get dropped when you learn all of the spells, but remains as a permanent part of your inventory. To join, go to the Fire Cave and defeat the Fire Lord (found in the cave west of the Dragon King's cave).
Enchanter: I haven't yet joined this guild, though the most powerful magic spell, Fairy Fury, is an Enchanter spell. Their weapon, the Mystic Staff, is the most powerful weapon Talia can use (but has to become an Enchanter to receive it). To join, go to Faiara and defeat the Enchanter. As an Enchanter, you will be able to call upon the help of fairies during battle.
I'd like to add yet another list that gives information on who can equip which item. I'd also like to be able to compile the equipment lists together into one, or at the most, two parts. Maybe I could create a more complicated table? Or abbreviate everything to a few letters to shorten up the columns?
I'd like to also consolidate the town lists so that each town has the sidequests that you get from each one (with where you finally go to complete it (if it's a multi-part merit quest) in parenthesis “( )”, as well as the items/equipment available from all the merchants. As you can probably tell, I consolidated the merchant inventories to one list, even if they sold the items by categories. Maybe in the future I'll split them up into the appropriate categories to make it easy to find an item, and to know exactly what to expect.
I'd like to expand the bestiary section to include more information (rather then the vague descriptions that I have there right now). I'd like to include attacks, spells they cast, items they drop, and fighting style. Most enemies don't have any weakness, so the strategy for going after any monster is to face it, and mash the spacebar until it dies. I'll probably end up removing the strategy part of each monster, and simply include any important information in the other sections.
Maybe sometime in the future, I'd like to include the merit (a.k.a. sidequests) quests in a separate section, along with which ones decrease your merit points, which one increase your merit points, and which ones don't have any effect on them. I'd probably also add a reference chapter for more information (so that the user can look up the sidequest they want to know about, then find it in the appropriate chapter so they can complete it).
Things that I'm missing
Not sure what the spellbook you acquire after joining the Enchanter's Guild, which has all the Enchanter spells to learn, is called.
Not sure exactly what Serenia says when she blesses either Devin and Talia or Devin and Alicia, nor do I know what Alicia's parents give you if you marry her and Devin.
I'm pretty sure that some of the items that I've placed a value of zero (meaning that it cannot be purchased) can indeed be purchased somewhere. Unless I find them, they'll stay just as they are. I think that placing a default value of zero is best.
Many thanks to the people who pointed out gaping flaws, and showing me that I am not perfect. It is a great lesson in humbleness, and I am grateful. My thanks also go to those who suggested improvements, things they liked, things they didn't like, and things they'd like to see in my next version.
Thanks to Patrick Mehaffey for suggesting that I should include cities with the side quests, and a few other minor details that I wouldn't have otherwise thought of.
Thanks to Maren for compiling the Masterguide. I “borrowed” (stole, actually, but as long as I give credit, I don't think I'll get sued) some of the information from that document to add some additional detail to my guide. Be sure to check it out if you have any questions on the various sidequests! You can download the Masterguide as a plain text file from my website, or view it as a series of posts from the Ahriman's Prophecy website.
Thanks Lindsay for giving me some information on what happens when Devin and Talia got married. I wouldn't've been able to add any information without those details.
Thanks to Cougar for the details on the marriage between Devin and Alicia. The direct quotes were most helpful!
The final “thank you” comes from the Player's Guide, from which I copied (again, I mean “stole” here, but I'm giving credit to show that I didn't compile the data myself) most of the data (I did verify it as I progressed through the game, however).