Much depends on the rules. For example in CT there is little advantage to Battledress over Combat Armor. In T20, there is lots to say in favor of BD over Combat Armor.
The big thing missing is a suppressive fire weapon. There isn't the equivalent to a Squad Automatic Weapon or Machinegun.
IMTU, (T20 rules) there are three branches of Marines. Line Marines (By far the most common), Ground Forces Marines (Used for Planetary security and similar missions where the Army isn't available or it wouldn't be politically a good idea to deploy an Army unit for one reason or another.) and Marine Commandos.
Ground Forces Marines are task organized and may have vehicles, such as APC's and may be equipped to a level other than TL-E+ (depending on era). These units are formed specifically for the task at hand and most Marines that serve there spend most of their career there. There are usually Marine NCO's and some officers, specifically senior officers that are transferred in from Line units. During the course of a deployment there will probably be some transfers out. (Though if you started there you are likely to need additional training to go to a "Normal" fleet unit.)
Fleet Marines serve aboard Starships, serve as Starport security (Usually as a Quick Reaction force leaving the mundane tasks to contracted locals, and Embassy Guards.) These are the Marines normally encountered and what most citizens and enemies of the Imperium think of when they think of Marines.
They specialize in rapid strikes from Starships, Boarding Operations, Seizing High Orbitals, etc.
Because of their reliance on Starship transport (After all the Navy would rather carry more missiles or supplies) Fleet Marines are largely light units. Because the environment they are likely to operate in is Vacuum and not on a planetary surface, they also tend to be light and instead of Tanks and traditional armored vehicles they are generally equipped with a specialized Light Fighter. (Multi-purpose makes more sense than specialized vehicles because of both lack of space aboard ship and not knowing where or under what conditions they will be deployed.
Line Marines are organized into a Squad of two 4 man fireteams. (8 men, which happens to be how many are on a Type-T.) Each Fireteam will have 2 marines equipped with Gauss Rifles, one with an FGMP-14 or 15 and one with a Light Tribarrel (Figure 3 Gauss Rifles welded together). Each Marine in the Fireteam is equipped with a special Marine Battledress which includes excellent passive sensors, laser comms, and a dual Grav/Leg drive. The Squad Leader leads the first fireteam and a team leader leads the second fireteam. At least one member of the squad is also trained as a Combat Lifesaver (Not quite a medic but almost.).
Three of these squads form the core of a Fleet Marine Platoon. The platoon will also have a Weapons squad. (Squad Leader and Team Leader with a Gauss Rifle.) This squad will be equipped with a mix of FGMP-14/15s and Tribarrels. (Mission dependent, as many as 6 of one.) The Platoon will have a Platoon Leader, Platoon Sergeant, and a Medic. (Total 35 members the same as the Marine complement of a Kinunir.)
A Company is made up of 3 Platoons, a 15 person Medical section. 4 Light Fighters (Crew of 2 and 1 member of a ground crew.)12 member HQ Section, which includes the Commander, the XO and a First Sergeant. (Total 150 members, the same as the Marine Complement on a AHL.)
I have it figured out to brigade level.