Hii Suhail.

The difference between avionics and mechanical aeronautics is:

Mechanical Aeronautics:

In this stream, all students must learn the basics of all flight system. If mechanical specialization is chosen, the aspirants are monetarily required to gather an in-depth knowledge and comprehension about the physical body of engines and every other details of about the parts of diverse aeroplanes that include:

• The heavy airplanes 
• The light airplanes 
• The jet engine 
• The piston engine 

Avionics:

These students are expected to gain a comprehensive and in-depth knowledge of diverse electrical, navigational and avionic digital system contained in any kind of aircraft. 

This specialized training comprehensively imparts hands-on and practical knowledge about all electrical system, instrument system, radio-navigation system, VHF communication modes, automation system including installation ,diagonostic, repairing and maintenance.

The avenue to choose mechanical or avionics depends entirely on aspirants depending upon their aptitude, inclination, abilities and capabilities.