I did not compare quality or whatever. If you define ogg as container or audio format it does not matter, as there are basically no external players which support it. Amarok+xine-engine+mp3 format = no problems at all. I will not convert any files from mp3 to ogg as reencoding degrades quality. The other way it is the same.
iriver devices or anything with Rockbox?
I have an old (2005) 1GB iriver fp-799 i think that has native ogg support.