What are the key differences between the Canon Speedlite 600EX-RT and the 580EX II, and which one would be the better choice for my photography needs?

The key differences between the Canon Speedlite 600EX-RT and the 580EX II are that the 600EX-RT has a higher maximum guide number, built-in radio transmission capabilities, and improved weather sealing. The 580EX II is smaller and lighter, but lacks the radio transmission feature.

If you need advanced features like radio transmission and better weather sealing, the 600EX-RT would be the better choice. If you prioritize portability and weight, the 580EX II may be more suitable for your Photography needs.