The difference really is that MP3, if you want to use it commercially, say the encoder/decoder, you'd have to pay for it. OGG is an open-source codec that allows a little more flexibility with the files, and I dont believe that you'd have to pay for it, or not pay as much, if you were going to use it commercially.
I store most of my MP3's in 192K format, some in 256K format, and I'll re-encode them down to a lower bitrate so they'll fit on my storage card for my Axim. I notice a difference when going from 256 down to 160 K but it's not a huge problem. That's just MP3, with MP3 to OGG, I havent heard a huge difference when transcoding like that. Though, an OGG and an MP3 at thier same respective bitrates sound different.