I just thought briefly I would point out some important variations between players:
The listed information below is not a direct terminology or concrete description of the design of classes. The information below refers to characteristics of performed players in game modes, but are able to differ.
Aggressive:
Mid to High level kill streaks
Run and gun play style
Rapid fire attachment
extended mag
Dual mag
Dual wield
Scopes
Sleight of hand
Marathon
Lightweight
Hardline
Shotguns
SMGs
Assault rifles
Light machine guns
Scavenger
Hardened
Warlord
Tactical Mask
Ninja
Assault rifles
Pistols or ballistic knives
Crossbow
Claymore
Tactical Insertion
C4
Support and Defensive:
Low level and reconnaissance kill streaks (spy plane, counter spy plane SAM turret, blackbird)
Sniper, camper or tactical teamwork play style
Suppressor
Scopes
Ghost
Hardline
Flak jacket
Sleight of hand
Scout
Steady aim
Hacker
Second chance
Ninja
Assault Rifles
Sniper Rifles
Light Machine guns
Certain launchers
Claymore
Motion sensor
Tactical Insertion
One is not better than the other but based off your game play and skill set. A player that dies often should utilize the characteristics of the support set to improve chances of survival by having the location of an enemy displayed on your mini map, jamming their mini map, destroying any air kill streak or better than spy plane. Have a real-time location of an enemy's location and facing direction displayed on your mini map.
The aggressive approach requires alot more awareness. Knowledge of map camp spots, spawns and high traffic areas. Knowledge of these areas increase the effectiveness of kill streaks. I recommend having classes adjusted to both and mixing them together. As the match shifts so will your play style giving you an advantage to surprise the enemy.