user47251: Which one is better to get started with register level embedded c coding?
Also which one is used professionally?

user490: Atmel Studio for professional. Arduino is for fun and is normally the first step into embedded c programming using the atmega328 (Arduino)

user25690: I actually wouldn’t recommend either for professionally developing for Atmel chips. Although Atmel Studio is a nice enough IDE (the simulator is amazing!), it does make it very difficult to unit test your code or to set up a build server. I would personally recommend you set up a Make (or possibly CMake) based setup directly on top of the avr-gcc tool chain and then simply use the text editor of your choice. 
As for learning about programming against raw registers, I have to recommend Atmel Studio and one of their XPlained evaluation boards. Atmel Studio includes a simulator that actually shows you, bit by bit, the current state of the registers. 

47251: <p>Which one is better to get started with register level embedded c coding?
Also which one is used professionally?</p>
 490: <p>Atmel Studio for professional. Arduino is for fun and is normally the first step into embedded c programming using the atmega328 (Arduino)</p>
 25690: <p>I actually wouldn’t recommend either for professionally developing for Atmel chips. Although Atmel Studio is a nice enough IDE (the simulator is amazing!), it does make it <em>very</em> difficult to unit test your code or to set up a build server. I would personally recommend you set up a Make (or possibly CMake) based setup directly on top of the <code>avr-gcc</code> tool chain and then simply use the text editor of your choice. </p>

<p>As for learning about programming against raw registers, I have to recommend Atmel Studio and one of their XPlained evaluation boards. Atmel Studio includes a simulator that actually shows you, bit by bit, the current state of the registers. </p>
