To me it really depends on what I draw first in terms of square. The best square I could find was a 3 x 4 x 2, and bordered by a forest and a river. I could make anything there, it will have amazing production, but I went Fortress. My next city was a 3x3x3 on a river. That will be a perfect Conclave.

I really like my Fortress to be high production because getting your troops quickly is important. I am playing as Pariden so I can have 3 essence there for buffs.

If you are not Pariden, your first city, well, it depends, Conclaves are really better with essences, so if you don't have a 3 essence square, I would make a Town.

I would have gone the other way around.  Essence is more important that materials for a fortress, and I usually make my fortress into my best overall tile.  You can add as many conclaves as you need, and their effects add.  You want the one single best fortress you can produce.  Given you are limited by crystals and iron in producing your best troops, and you can rush with Gildar or "Call of Duty" essence bonuses matter a lot more than production.

Conversely, conclaves need a lot of production to build up.

My rule of thumb is: My fortress goes in the tile with the most essence.  I will delay getting a fortress as long as I can unless I can get a 3 essence tile.

I usually go with Town, Conclave if it has 2 or more essence. Cities whose zone of control do not join up with your capital cities zone of control have an unrest penalty. I find the extra zone of control distance that a Town provides is useful in this case.

First Settlement is a town unless I have both high materials and essence, than it is a Fortress.

It seems the most common tiles are 4/2/2 or 4/2/1.  If that is the best I can get for my starting location than it is a town.  I have started thinking the 4/2/2 would be good conclave for a first city.  Try to maximize my research.

My general rule is I would rather settle a 2x3x2 tile than a 3x2x2 tile, I put mats over grain unless it is an extreme difference like 5x2. An essence is, by the end of the game, worth more than 1 food and 1 mat, that's because you can get mana per essence, 25% bonus food per essence, mats per essence, gold per essence and research per essence buffs. There might be a grain per essence buff but I always unlock the are/water spell for the bonus food instead. Some of those are pretty late in the research tree.

I guess I kinda cheat in this sense, as I will generally ctrl-n until I get a starting location and yield I want to play a game on. That tends to be 3 essence, because I like the huge effect per essence spells like propaganda/arcane forge provide. I've been picking conclave as my capital just about every game also. High essence for conclave turns into a really nice early mana income, and I like to play spell caster sovereigns.

Although, I can certainly see why having a high essence fortress would be useful. With the right enchantments on the city you can pump out some elite trained units.

If I'm playing a magic intensive faction, then my first city is usually a conclave (and my second and my third...).  If not, then the first city will either be a fortress if it has good materials (3+) and essence (2+) or town if it's near a river and/or ho-hum materials (<=3) and essence (<=1).  Clear as stinking mud?