Resurrecting with another questions after further research. Out of SRAM's new 12-speed line, I am liking the 10-50T Eagle (XG-12xx) series. If I were to choose one of the Eagle cassettes, would I need to get specific Eagle derailleur? What options would I have then if I wanted the Eagle cassette with eTAP for a road bike with drop bars?

Also, what can you tell me about free hub compatibility for SRAM cassettes? Will the 12-speed lineup fit on XD or XD-R? What is the real difference between the two? Can I just fit them onto Shimano hubs?

Thank you!