Between the AGMG and DMGII, I think I prefer the AGMG. If you are a modest to intermediate GM, the DMGII probably has more for you to "take it to the next level". The AGMG has a bit of the same style of advice, but if you have already read Robin's Laws of Good Gamemastering (which overlaps a bit with the first chapter of DMGII), you might find the perspective of AGMG's advice fresher. The AGMG is a bit more about experimenting and expanding on your gaming experience and has more pure utility material and variants that will be of more use to more advanced GMs.
Here's a recounting of the content of both books and how useful I personally consider them to be (make your own judgments, natch.):
DMG II
Useful
- Play styles and player traits (though it will be old hat if you have Robin's Laws of Good Gamemastering)
- Other GM advice
- Archetypal Locations (love this sort of stuff)
- Political plot hooks & courts (though some of this is old hat)
- City and town plot hooks
- 100 instant NPC agendas
- Magic events (but R&R's ritual system fills this niche for me pretty well)
- Saltmarsh (well done and useful)
- Contacts (sort of)
- NPC unique abilities (sometimes the D&D system can make things a little too predictable.)
- Business rules
- Designing prestige classes (we've seen it before, but IMO it's essential)
- Signature traits
- Bonded magic items
- Magic locations (iffy - not sure if I'd use it. Too dependant upon PC interest.)
- Synergy abilities - just barely said to myself "I wish there was something like this." That's usually a good sign
- Magic Item Templates (iffy, but used right can have interesting flavor)
DMG II
Not so useful
- Extra traps (sorry, dull and I already own Song & Silence and Traps & Treachery I&II. Book of Challenges is much better IMO.)
- Miniatures & Battle Grid Notes
- Encounter Tables
- Campaign Building (unless you are really a basic GM)
- Town minor NPCs & establishments
- Modeling law and justice (at one time I would have dug this; I have no time for this level of emulation before, and I certainly don't keep the last 100 years worth of transitions for every city.)
- Complex NPCs (one or two might be useful. They need the DMG 3.0 ready to run stock NPCs back; Ultimate NPCs also fills this niche much better.)
- Mentor/apprentice rules - interesting idea, not compelling enough.
- PC Organizations - don't think I needed rules or guideance here
- Guilds - likewise.
- Teamwork (blah)
- Artifacts (again, old hat)
AGMG
Useful
- Sensitive Subjects - too many GMs are NOT sensitive to their player's, um, sensitivities.
- Knowing your group / play style - similar to DMG II, but might be fresher perspective if you have read Robin's Laws before.
- Combat rules variants - esp. class dodge bonus
- Speeding play
- Hex based movement - my preference for outdoors (iffy, though, cause it's old hat)
- Magic rules - esp metamagic points and self limited spells.
- Preparing adventures
- 40 basic plots/40 fight scenses/100 urban details / 100 wilderness details
- Simplified NPCs
- 40 campaign themes
- Cosmology choices
- Tweaking campaign rules (feats/bonus points)
- Designing feats & PRCs
- PC power level variants
- Ability score generation variants
- Training variants
- Innate abilities - AT LAST! Lots of great ways to reward players without magical bling.
- Levin (power component rules done justice, and eschewing the permanent "butcher shop" mentality.)
- Sovereign materials - appeal to the munchkin in me
- New special qualities - farms out those nifty abilities formerly locked up in special weapons.
- Inconveninces
- Mystic Locales & Holy sites
- Initiative cards (though I already use these)
AGMG
Not so useful
- World Design - old hat, for the most part
- Play environment - old hat
- Fortune points - sorry, AFAIAC, action points are the standard
- Designing antagonists and allies (might be useful for newer GMs)
- APM NPCs (which I don't use any of except eldritch weaver)
- Character backgrounds (would never use these)
- Calendars and events (too simple, wouldn't use)
- Advancement options (none struck my fancy)
- Item characteristics