For most MP3's, variable bit rates (VBR) have the best of both worlds - small size due to increasing/decreasing bitrates as required.
All my mp3's are in VBR (variable bit rate) "alt preset standard" setting using a recommended lame build from hydrogenaudio.org. They can go as high as the max 320kbps if necessary and right down to the min at silent parts.
They extensively tested its performance using top equipment and involved many people from the forum in listening tests before arriving at the final settings.
For Amiga tunes though I'm not sure how viable VBR is due to the 4 channel limited nature of the tunes, CBR might even be better. Those ones on your site seem fine to me, but if you want some great advice post at hydrogenaudio and they'll soon let you know!