Assuming you are not spawn capped and have a capable puller, adding a 6th is almost always a good option.
First off, the healer isn't going to be doing much if any dps, the tank does minimal dps relative to dps classes.
So let's say a healer + tank does dps at a rate of 1.0. Giving a damage rate of .5 per person. If a dps does 2x the damage of a tank that goes up to 3.0 or 1.0 per person. A second dps brings that up to 5.0 or 1.25x a person.
Obviously, this is rudimentary guideline, a lot more factors must be considered. Which is why in practice 4-5 man groups are often more efficient than 6 man.
But I have to say, assuming things align, a ranger as your 6th slot is going to increase your xp gain, especially if your tank is a pally or a sk and you have someone spending most of their time pulling
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