I swapped from RCD510 to RNS510. It will have a camera input (only some, not all RCDs have that) as well as the NAV. The traffic is different too - holds a lot of bulletins for you to scroll through. Re the Phone: both RCD and RNS can have same phone set up. If you get a later (or new) RNS; it will also have voice control function - but you need the BT mic obviously.
However; some minuses: no Radio Text, and you can't program it to record morning / pm traffic bulletins from a designated station as I used to get my RCD to do. Still; the nav is the best I've ever used, by some margin, and the camera (if you go for that) is very useful too.
Hope that helps.
dieseldunc