I am wondering how will these two differ, One is Ac to DC converter module, second one is SMPS.

Link1(AC to DC module)
Link2(SMPS)

when link 1 is available in a small size than Link2(SMPS). Under what circumstance we have to use Link1 and link2.

I understand that SMPS is 5watt, Link1/Module is 3 watt.
I am looking for the technical differences.

 

LInk2 is rated at 5V @ 5A = 25W. That's why it's considerably larger and designed for better cooling.

 

There will not be much difference between the two, technically.
Link 1 is compact, maybe with the pass transistor built-in into the controller IC.
Link 2 is with external pass transistors.
As mentioned in #2, the difference is mainly the powe handling capacity.