What helicopter is it? It's not a matter of choosing which one you want to use, it's which one does your helicopter use.

In a fixed pitch helicopter, the blade grips and blades never move. The blades are built with a certain ammount of pitch much like an airplane propeller, and thus, the faster the main motor and the rotorhead spin, the more lift is generated.

A collective pitch helicopter is more complex, however, as these are how real helicopters operate, and is the "true" mechanics of a helicopter. These helicopters use blades that have no area of attack (pitch). Instead, the motor typically runs at a steady RPM, and the blade grips and blades them selves rotate in flight to produce the area of attack and subsequently, lift.

If it's a fixed pitch helicopter, then you need to build pitch into the blade, or atleast produce a flat bottom airfoil (semi symmetrical) with a twisted root, in order to produce the lift to get the helicopter off the ground.

If it's collective pitch, then it's just a matter of finding an efficent airfoil design, and then deciding on the type of flight you're interested in. Flat bottom blades (semi-symmetrical) produce good lift and stability in normal upright flight, but don't work well if you flip the helicopter or try very fast forward flight. Symmetrical blades use the same airfoil shape on both sides of the blade, which makes it well suited in both upright and inverted flight and thus better for aerobatics, but not as efficient as flat bottom blades would be if you just want to hover around.