I had the eu rom on and when I checked the site and sammobile there was a direct update, however kies nor ota picked it up. Could be sim related but I just grabbed the SA rom and updated with odin.
Also ota does a periodic check, honestly not sure when or how often its done, kies os obviously when you connect. But weather you do it ota, kies or odin you still getting the exact same rom and will be downloading the exact same file. Both open and vodafone named roms on sammobile have identical pda and csc codes and are both marked as available through kies.
Its an official update so no matter which update method you use you will still have your warranty, you will still have all your app, configs and what nots installed. Only thing is if you used root you will need to re-root.