Everything you need to know is on the Ross-tech website.
I believe you can only use the HEX-CAN cables, whether it be Serial or USB.
See the application chart at this link:
http://www.ross-tech.com/vag-com/cars/applications/a5s5.html
Yes, HEX-CAN is the one you need - it covers both old-style K line diagnostics and newer CAN diagnostics. A4 B8/A5/A6/A8/Q7 etc. need both. Last generation A4 needed only K line (so chances are your old VAG COM cable is not up to date for A5) and e.g. A3 needs only CAN diagnostics (hence the MicroCAN option on Ross-Tech site which only works on A3 and some other VAG cars, but not on other Audis if I recall correctly).
MOST or not MOST doesn't really change anything, at least in MMI 2nd gen MOST components still use K line for diagnostics and MOST only for other data/media transport. Some components in an A5 use K line, some CAN.
But for A5 you need HEX-CAN and it works on the most cars anyway (because it covers both diagnostics technologies), so it is the most versatile and future-proof purchase too.
I updated these notes to my VAG COM article too:
Interesting VAG-COM codes for Audi A5
http://www.geocities.com/a6retrofit/articles/a5vagcodes.html
