Spellstone or Firestone?
You say you are playing in the demonology spec at the moment, and as it happens, applying your weapon buff is more important to demo locks than the other specs. In raiding trim (a good baseline), the stone will account for about a 4% DPS increase for demo locks but only half that for affliction or destruction. The rule of thumb is to use a Spellstone unless you are destruction. That said, at lower gear levels the difference for destro is marginal.
Given the two mechanics of each type of stone, it's interesting to see which part is of the most benefit. We're in an unusual time with high-end raiding, as all three specs are very viable; as such, we can draw comparisons between all three. For destruction, both the crit and the direct damage boost offer about the same DPS benefit. Affliction gets a bigger boost from the haste component of the Spellstone, whereas demonology gains significantly from the periodic damage boost (far more than the haste).
Bear in mind, though, that these differences are all fairly small in the scheme of things. You will see far more variance in DPS results through rotation changes and situational eccentricities. That in mind, the rule of thumb is perfectly serviceable.
Fel Armor and Demon Armor
On the face of it, this is pretty easy to answer: Fel Armor is the one to go for in pretty much every situation. In a group, you are there to do damage, and this is a significant damage boost. At tier 9 gear levels (entry level for Icecrown Citadel), you lose about 10% of your DPS if you forget to put up Fel Armor.
That said, Demon Armor has its place. Every so often, a warlock is called on to be a spell-tank. This means that there is a mob or boss that doesn't hit hard (or at all) but throws a load of spells, and the group want the warlock to tank it. There have been a few raiding instances of this in Wrath, and there were more in TBC. In this case, you typically want to have Demon Armor on. This isn't for the armor bonus but so that the healers can more efficiently keep you alive.
I've heard the "art form" explanation before, and I'm never satisfied with it, either. It would be more honest to simply say, "It's complicated" or "I don't know." Essentially, what you have is something very similar to a trinket effect. You have a damage boost (and some extra abilities) that you can use for a short time and are on a long (ish) cooldown. The simplest way to look at getting the most out of it would be to see how to spend the most time in demon form. So you would look to use the ability as soon as possible and again as soon as it is off cooldown.
You then start to think about other things that might make you change this approach. What about when Herolust is activated? If you can get meta to coincide with that, then it will be an even bigger boost to your damage. Should you time it to work with your on-use trinkets? Sure, you would want to, but if they have different cooldown times then they become out of sync. Will you have a full meta duration during the execute phase? Depending on how long the fight lasts, you might want to time meta so that you get as much time as a demon while Decimation is active. Will you be able to make good use of Immolation Aura? ImmoAura is a very nice bit of extra damage but is far more effective if used against more than one target; if the fight offers a chance to use it this way, you might want to time meta for that. Do you have maximum damage potential at the start of the fight? If you have to wait for a trinket to proc or buff/debuff to become active, you may well want to delay popping demon until it is.
These are some of the primary considerations for getting more out of meta, and they really do make the decision complicated. Not only are there things that can mean you get more damage by moving away from popping meta as soon and as often as possible, there are also things that will drop your DPS -- such as being carried off by a Val'kyr on Lich King. You need to make sure you don't pop meta and then lose the bonus damage by having something bad happen to you, or a phase change that drops the worth of the demon form.
So, yeah ... I don't know. It's complicated. But with time and experience, you will start to be able to assess the various factors quickly and make more use of metamorphosis.
Blood Pact is a weekly column detailing DoTs, demons and all the dastardly deeds done by warlocks. If you're curious about what's new with locks since the last patch, check out WoW.com's guide to patch 3.3 or find out what's upcoming in Cataclysm from the BlizzCon 2009: Class Discussion Panel.