I found this illuminating essay on the differences of Phalanz vs. Legion. It is also true that under the marius reforms the Cohortal legion was formed which coincided with the time of caesar.
The Phalanx Legion: The early Roman legion was very little in terms of organization than its Greek counter-parts. It was a massive, deep formation of soldiers with interlocking shields, armed predominately with spears. The spears were pointed outwards in front of the formation, ready to mow down any soldiers who dared oppose it. The phalanx is essentially a mountain, extremely slow but nothing can withstand it. The core of the formation are the rich landowners, as in ancient societies often they were the only ones who could afford the armor and equipment required to fight in the phalanx. The majority of phalanx-using populations fought as skirmishers rather than heavy infantry. However it has major downfalls. The phalanx is solely designed for attacking the enemy from the front of the formation, the disadvantage is the tight formation restricts movement heavily, it takes a very long time to wheel around to face new directions, making it suspectible to flanking manouvers. In bronze age times often times two large forces at war would agree on a place and time to battle, and both sides would line up on either side of the battlefield and advance. For this the phalanx was perfect, however the Romans often times in the early Republic did not fight organized enemies with whom they had a predefined battle site. Largely because of this purpose, a new form of organization was required, which most believe was introduced gradually over a long period of time.
Manipular Legion: Throughout most of the Republic period this was the force used by Rome. It consisted largely of a three-line checkerboard formation, designed to give their enemies a two-punch killing blow on the field of battle. The primary unit of organization was the maniple, the amount of soldiers in which has often been debated, as some historians believe roman numbers fail to include cooks, blacksmiths and other essential personel which might be attached to the unit, but this summary is based on that a maniple consists of around 160 fighting men on the field. The maniple is then divided into two centuries, each of which consisted of 80 men. When deployed on the field, the legion consists of three main lines, around which the plan of battle is constructed. This organization appears to almost entirely have been based on age. In the front were the hastati, the most inexperienced regular soldiers of the army. They were generally equipped with body armor covering the upper body, a large square shield, a javelin, an iron helmet, and the gladius shortsword. Behind the hastati were the principes, who were similarily equipped only with weapons of slightly better quality. Also, the principes were slightly older and had more experience. Behind them were the triarii, the oldest veterans and the last remnants in the legion of the Greek phalanx. They were equipped differently than the others, having heavier armor and a different primary weapon, a large spear called the hasta. Also the triarii maniples might have been slightly smaller in size, having about 130 men per maniple rather than 160. In addition to this, there would be about 300 horsemen and a large assortment of velites (skirmishers). The formation would be arranged much like the following, with the hastati in front, principes second, and triarii third, with each - signifying a space between maniples. Each [ signifies a century.
[]–[]–[]–[]–[]–[]–[]–[]–[]–[]
–[]–[]–[]–[]–[]–[]–[]–[]–[]–[]
[]–[]–[]–[]–[]–[]–[]–[]–[]–[]
The velites would be out front in a loose formation, armed with javelins, and would usually skirmish with the enemy before the main battle. After the skirmish was over, the hastati would attack first, throwing their javelins and then charging to meet the enemy. Next, the principes would attack, throwing their own javelins and filling the gaps in the formation. The triarii would remain in the rear, if the hastati and principes were thrown back they would cover their retreat, if not, they could be used to prevent flanking maneuvers by the enemy or launch some of their own. All in all, excluding velites, the legion had about 4800 men.
Cohortal Legion: The Cohortal Legion is called so because of the same reason as the Manipular, the main unit of organization was the Cohort. This legion, like the previous, was a result of a large number of reforms over the army, however those for the Cohortal legion took place in a much shorter period of time and many them were instituted by the same man, Marius. The process of creating a Cohortal legion had already began some time before, however, and legions with a Cohortal level of organization had already been used before Marius. The main difference was the form of recruitment. The Cohortal legion was one of the the first professional armies in history, all of its soldiers having been recruited, trained, and equipped by the state rather than their own means, negating the importance of the nobility in the military. The Cohort was a unit of organization larger than the maniple, consisting of about 600 heavy infantry soldiers, divided further into 6 centuries each led by a centurion. The legion all in all had about 6000 soldiers, around 10 cohorts. Each cohort was a small army in and of itself and could break off and perform military manouvers without the rest of the army, which was one of the greatest advantage of the legion. For instance, if the main force was engaged, a cohort could break off and perform a flanking manouver, an action previously impossible with a phalanx and difficult with the Manipular legion. Like the maniple, the Cohortal legion was organized into a checkerboard pattern, to allow the filling of gaps but also for each line to form together in a single front if need be. This time a + signifies a gap between lines. A set of -'s signifies a cohort.
–----++±-----++±-----++±-----
++±-----+++±-----+++±-----
+±-----+++±-----+++±-----
Generally, each cohort would be about 100 ranks wide and 6 ranks deep, with each century forming a line. Usually the first cohort in the formation (the one on the front left I believe) was the most experienced and being placed in the first cohort was much like being placed along the right side of a phalanx, it was a position of honor. The soldiers themselves were armed much like the hastati and principes of the earlier legion, with a javelin (or pilum), a large square shield, a gladius, and armor covering the upper body. Auxileries (cavalry and missile units) were generally not included in the legion count.
Conclusion: The Marian reforms helped create one of the best fighting forces in the world by the end of the Republic. However, as cavalry of all sorts began to once again dominate the field as they had in Alexander’s time during the Imperial phase, the heavy Roman infantry gradually began to evaporate into a lighter style of sloldier armed with a small spear and shield. The infantry had to become lighter in order to keep up and cope with the cavalry, and by the fall of the Western Roman Empire, the legionary soldier was almost in no way identifyable with the Cohortal soldiers at the end of the Republic.[/quote]
This esssy does illustrate how the legion especially the Cohortal Legion was vastly superior to the phalanx and secondly the concept of a professionally trained army raised by the state and did not recoqnize the importance of nobility which is a problem that plauged armies of both sides during Alexanders time. I understand that this does not have any bearing on how Caesar was better than Alexander other than to say that his armies organization was better in many ways.