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How can we compare between the subject techniques in terms of following parameters:
1. Capability of detection of defects
2. False Call Rate

This would help me in deciding a suitable NDT for huge welding scope.

See the figure below which provides a complete comparison of all the available techniques for welding inspection including TOFD, Pulse Echo Meander Scan, Pulse Echo Line Scan, X-ray, Gamma Ray and Manual UT.
You have to make a careful decision in picking the right one for your application.

Ashfaq,
Thanks for the crisp information.
Can you please let me know about the limitation of UT techniques?

Limitations of UT techniques involve:
1. Thickness (TOFD can scan (good) only down to 12 mm - Pulse Echo can go upto 6 mm as per EN standards)
2. TOFD requires atleast 300 mm clear distance on both sides of the welds. Therefore, weld joints with flanges, valves, reducers / expanders cannot be scanned. Additionally, TOFD is also not practical on welds below 6" and on welds with bends below 8" / 10".
3. Pulse echo meander scan takes much of time for scanning.
3. As mentioned in the figure above, manual UT has the highest False Call Rate. Manual UT is time consuming also.

I suggest that the going in position for UT shall be to adopt a combination of TOFD and Pulse echo to cover all the scope.

good explanation by Ashfaq.

Ashfaq,
Nice & comprehensive details. Pls comment that the comparison data provided is establised thru experience or it has been given by some vendor/service provider for these techniques?

The comparison is based directly on the experience over the years with these techniques.
Even the vendors would be giving you the same inputs on the comparison shown above.

The graph provided by Ashfaq comes from a study by the Dutch Welding Institute (NIL) - Stelwagen, 1995. Its often shown in the literature, e.g. Charlesworth and Temple - Engineering Applications of ultrasonic time-of-flight diffraction 2nd Edition, Fig. 8.14.