Chain healers {def, myst} use wards as a first line of defense. Wards absorb damage. Plate healers {temp, inqu} use reactives. Reactives heal for X amount on damage for Y amount of triggers. For example, the top tier spell :

Chain - Ancient Aegis - Wards target against 1575 points of all damage
Plate - Vital Intercession - Heals target for 394 - 481. Grants a total of 5 triggers of the spell.

The wards prevent damage from getting through, whereas the reactives your target still takes damage. With a large hit on a tank, a ward will absorb most if not all of it. A reactive, the tank will essentially take the damage and regen a limited number of HPs. So, a hit for 1600 would eat the ward and spit out a small amount of damage to the tank, leaving your tank "naked" untill you can recast the ward {via the 6sec cooldow or alternate wards}. The same hit on the tank with a reative would do 1600, heal ~450 and continue to heal ~450 the next 4 times the tank gets hit, so if the tank gets hit for less, the reactives could feasably "catchup" the HP defecit {especially if you layer your reactives}.

With certain tanks, monks and SKs come to mind, I personally feel wards are yummy. SKs lifetap enough that by the time a ward is broken, it's up again and any damage that gets through has been regenned through a number of other means {SK spells, melee procs to group heals, involuntary healing debuffs and wards}. Monks are avoidance tanks, with less for mitigation should they get hit. Wards absorb damage that they would take instead of having them "go against their nature" and needing to be hit to make use of reactives.

Basically, they are different ways of healing. Plate healers can work just as well as chain, especially if you can get some other forms of HP regen going. It's mostly about what you feel comfortable with...being able to negate steady damage at a consistant rate {reactives} or being able to void out a set level of damage at a fluxuating rate {wards}. Being able to both is why I run 1 plate and 1 chain.

As far as 6 v 4, technically, I'm 5 +1. My husband covers all the tanking stuff, I take care of the rest. We do anticipate that at some point, our friends from WoW could come back to EQ2 with us. We also anticipate that any other member that we add to our groups will be DPS. We have tanking, healing and utility covered, DPS is easy enough to mix and match while still keeping a good amount of synergy. We haven't done too much with the high-end dungeons, but I imagine that it's a lot like WoW...know what's comming and set it up beforehand. Other people coming along only makes it easier for me with less to micromanage. Having all the best buffs at hand is the hard part, and having the core of that by picking up the classes other people don't like to play {but make my 6 team ideal} resolves that issue.