Hii aspirant
It is up-to you to decide that what is better for you on basis of your passion and interest
Today with air traffic increasing there is good scope for aircraft maintenance engineering
Aircraft's needs both mechanical as well as aviionics it depends upon you what you like and what kind of work you want to do
I can just tell you about them
MECHANICAL
in this specializations students gather in depth knowledge about the physical body, engine and every other minute details about parts or different component of planes
AVIONICS
In this specializations you gain knowledge about
Electric system, instrument system, radio navigation system, VHF communication models, automation systems, which includes installation, diagnostic and prognostic maintenance and repairs

Aircraft maintenance engineering is not a degree/diploma course. It's a training programme. On the completion of the training, a licence is issued by the Directorate General Of Civil Aviation (DGCA).
So as per my knowledge, any field can be opted.
As you have to go for training programme.
For further queries you can check link given below.

https://www.google.com/url?sa=t&source=web&rct=j&url=https://engineering.careers360.com/articles/aircraft-maintenance-engineer/amp&ved=2ahUKEwjHy7_5kJ7iAhVEU30KHU6pB6cQFjASegQIAhAB&usg=AOvVaw2EutqCc2gyzRA2Ei7U2rcR&ampcf=1

Hii,If I talk about Aircraft Maintenance Engineering then you will have the option to continue it in either MECHANICAL stream or AVIONIC stream. On the other hand only mechanical engineering I don’t think so able to suppose the skills of AME. So it can be suggested that if you are interested in Aircraft Maintenance Engineering then you will have to option to continue it in Mechanical branch.From the career point of view if you have done AME in Mechanical you will no doubt have job options in both mechanical as well as AME engineering. On the other hand if have done engineering in mechanical branch then it can’t be said you can go to Aircraft industry.

So in my opinion you should choose Aircraft Maintenance Engineering in Mechanical. It have a great scope of jobs in and outside India.

Hope,this information will helpful for you.