It depend on what type of city you need in that moment.

If I'm looking for a Conclave, I'll go for the 3/3/2, if I need a Town, I'll go for the 2/6.

In the particular situation you describe, it would, not knowing of other factors, be a tough call. However, essence is generally superior to grain and material. It depends on a number of factors, though, such as what spells you have available to cast on a city, what your overall situation is etc. Also, the later in the game you are, the better essence becomes (due to spells such as Arcane Forge).

Essence is generally extremely powerful, probably even too powerful. Beyond the other bonuses a city with essence offers it can almost always make up for the 'lost' grain/material through spellcasting while usually also having room to net you a large chunk of mana. I do believe a rebalance may be required.

I would almost always go for the essence location due to its flexibility.

If you need a Town, I'd take the 2/6/0.  If you need a Fortress or Conclave, I'd take the 3/3/2.  

Towns just don't really benefit much from Essences.  Sure, you can spend 110 mana to make it seem like a 3/6/0, but chances are good that I don't have 110 mana to spare.

Fortresses need Essences.  If it's going to produce units, you want enchants that improve the units, because they make a big difference.  If you just want the level-up bonuses, the enchants to improve growth are key. 

Conclaves rely heavily on Essences due to the Herbalist upgrades (the Herbalist itself doesn't, just the upgrades).  

It should also be noted that some buildings require the city to have at least one Essence to be built, such as the Cleric.

I would revise this to say that "Troop-training Fortresses need Essence." Not all of the fortresses I get are needed for troop-production - some I get for map control, some for unrest reduction, and some for building armies, but not all of those necessarily go hand-in-hand. If I already have one or two Fortresses with decent amounts of Essence, I don't really need another one, so after I have my one or two high-essence fortresses I'm not going to put down more fortresses on high-essence sites unless I decide I need more troop production.

I'd tend to agree that towns don't benefit too much from essence, but it depends on what you put into the town. Propaganda and Arcane Forge on a town specifically built for wealth production are great, and extra food on a town built for rapid growth is useful (but only for feeding the sacrifice spells), and any spell that boosts production is always useful on any city. I'd still rather use essence sites for conclaves and training fortresses, but you only really need so many of those, and once you get to that point you might as well shift towards towns instead.

As for how much it's worth? As a rough estimate, I'd say 2 essence = 3 of any combination of grain and materials, but that's going to vary depending on my own needs and what my game strategy is.

I forgot to add that I consider forests to work as something like a 50-100% materials bonus, since it allows you to double up on production buildings which have similar production-per-material bonuses. So I'd rate your 2/6/0 with forest access as something between a 2/9/0 or a 2/12/0 rather than a 2/6/0. River access is more of a gold and food bonus, which matters more for Towns than for Conclaves or Fortresses.

Since you said that the 2/6/0 is "in a forest" and that the 3/3/2 is "just next to it", the 3/3/2 probably has forest access. In that case, I would be more likely to take the 3/3/2 than the 2/6/0 unless I'm looking to build a Town, as the two sites would then be roughly a 2/9-12/0 vs a 3-6/5-12/0-2. If the 3/3/2 does not have forest access, then which site I favor depends much more on what I'm looking to build.

Regardless, though, the value of essence as compared to grain and materials is highly variable depending on what else I have in my empire and what my game strategy is.

Depends on what the other spots that I'd be giving up have. I've passed up 3/3/3 sites for a couple 1/6/0 or 4/2/0 sites before when I'm more interested in money, production, or map control, and I've also passed up taking multiple halfway decent sites for one really good site. There isn't just one answer to these questions you're asking - multiple x/y/0 sites are probably better for your gold income than a single a/b/3 site (although with the reversion of the Propaganda nerf, that might no longer be the case), and having lots of towns floating around can let you cut back on the infrastructure of your Fortresses and Conclaves quite a bit, which is useful since the less time you need to spend building production facilities and food structures, the more time you could be training troops or producing research or something else, and you can start on that earlier than otherwise. On the other hand, if I need a troop-producing fortress, I'd take an a/b/3 before an x/y/0, and I'd rather have at least one essence on any Conclave I build (although that can be added by an Oracle, so natural essence isn't essential, unless I really want a Scroll Scribe or Archivist).

To me, 1 essence is more or less worth 1 grain or 1 material, 2 essence is worth 3 of some combination of grain and materials, 3 essence is probably worth 5 total grain and materials, and a 4+ essence site I'd almost take regardless of anything else in the area because it's a beautiful site for a troop-training fortress and because I see such sites so rarely in my games. It still depends heavily on what else is in the area, what else I have in my empire, and how I'm playing that particular game, though, so while these reflect how I usually decide on cities, they don't reflect how I always choose my cities, nor do they reflect how I always play, because sometimes it's fun to go for empires with lots of cities, and sometimes it's fun to go for empires with really good cities, and sometimes it's fun to go somewhere in between. I'd rather not get to a point where I do have a hard-and-fast rule for picking city sites, because then I'd probably end up playing the same way every game, and that would get to be boring.

Hmm, if you start with life magic, and you should (heal is just too powerful to forego at the start), then with Kingdom, at least as Pariden you start with +1 mat spell, +2 Growth Spell, and +1 research per essence spells. So, you can see, three extremely useful spells right as you start the game. So, 2-3 essences is huge. I don't know if Pariden gets a special spell there or its all life magic, but that grouping of spells, none of which cost much to cast destroys any other tile.