TL;DR: Two partners are always more efficient than a group or committee.
In the original cartoon & comic, the Headmaster technology was developed on the planet of Nebula. The technology allowed a Nebulan (or human) to receive several cybernetic transplants, thus allowing them to combine with a Transformer in a process called "Binary bonding". The process of binary bonding allowed the two minds to work in tandem, with some basic functions being "offloaded" to the Nebulan/Human mind, while offering the Transformer access to the more unique qualities of the human mind, such as intuition and "sixth sense".
The process was not perfect, nor was it seamless. The binary bond was most effective when the Transformer and its partner possessed personalities that synchronized well. If the Transformer possessed an aggressive, warlike personality whereas the Nebulan/Human possessed a passive refusal to kill, it would present problems. In such a case, the minds would conflict, causing the combined body to freeze or hesitate while the two minds argued over which course to take - in effect, the same exact problem seen in gestalt Transformers.
The best analogy for a Headmaster team-up is a marriage - when the two partners are in sync, it can be extremely effective.
In nearly all versions, gestalt Transformers are formed when a team of five or more Transformers combine to form a new Transformer. Rather than creating a new mind, however, gestalts are actually a "link" between the individual minds creating the body. In such a case, a gestalt mind is literally five or more minds attempting to work in tandem - as long as they all agree to do something, the body does it.
The obvious drawback to gestalt technology is that, even under normal circumstances, it can be difficult to get five Transformers - especially Decepticons - to agree on a specific course of action. If a gestalt Transformer is presented with five separate targets, and each individual within the gestalt wants to attack a different target, the end result is that the gestalt either freezes or lurches uncertainly towards one target or another.
The best analogy for a gestalt team-up is a committee, or any other room full of people with equal authority - for every action, the first task always has to be agreeing what that action will be. The second task is to agree on HOW to perform that action, and so forth.
BTW, gotta give credit to @Izkata for the committee analogy on gestalts.