I've dome my Masters with CS . I wannabe iOS dev what should i do first. Please explain me Sir.

If you are wanting to learn Swift to build iOS apps, then both can be followed. If your wannabe iOS developer status means you might be working on existing projects then following the original 100 days course makes a lot of sense as it uses UIKIt which is still the basis of most iOS apps out there. If you are going to building something from scratch then that would also work, but the wind if blowing towards SwiftUI being the future for Apple, on all their plaforms and not just iOS.

These is no right answer that suits everyone, but if you have no other reason to make a decision then SwiftUI is the new shiny.

Hope that helps.

Thankyou so much @tartanhare for help to Understand things.