What is the main functional difference between a pneumatic diaphragm actuator & a pneumatic piston actuator while operatin a globe valve.Will it be difficult to attain the precise control with piston actuator?

Diaphragm actuators evolved when valves were operated directly with the air signal from a controller. Nominally the signal range is 3-15 psi. 

It is then necessary for the actuators to have large diameter to genrerate enough force to move the valve. 
Diaphragm actuators are sensitive when considered in an open loop.i.e.: you will get a small motion from one with a small change in signal.  However in a closed loop (attached to a valve) they are not as ideal.  Packing friction in the valve is then the limiting factor for sensitivity.  A large-diameter low pressure actuator lacks stiffness (Think "waterbed") so a small incremental change in air pressure is less likely to overcome packing friction and conversely process pulsations are likely to be able to compress the low-pressue air in the diaphragm chamber so the valve can be moved by the process. 
Positioners help, but the air pressure is still limited typically to 60 psi (4 bar) and the stamped metal case, diaphragm plate, and rubber diaphragm all deflect with changes in pressure. 

Compare to a cylinder: The cylinder uses a positioner and much higher pressure air-typically 80 psi(5.5 bar) , up to 120 (~8.5 bar) .  High-pressure air is stiffer.  Cylinder actuators have smaller diameter because the higher-pressure air generates as much force against a smaller area. The friction of the piston seal in the cylinder actuator is still no greater than the packing friction of the valve. If you send the positioner a small incremental change in signal it meters in air to the cylinder at up to full supply pressure until the valve moves to a new position corresponding to the new signal.  With the smaller volume of a cylinder, if the process pulsations push back on the valve plug, the change in air pressure (force) per amount of deflection is greater than a diaphragm, particularly (exponetially) at the critical positions near the ends of travel.
The piston is very rigid compared to the diaphragm plate, diaphragm housing, and relatively stretchy diaphragm.  
With less volume in a cylinder it is able to respond more quickly.
 
Yes,  it is very possible to attain extremely high resolution control with a cylinder.

MORE NEWS