the namimg scheme in iDVD was a bit .... misleading.
best performance - CBR-encoding, gives best (!!) overall results in terms of quality
high quality - VBR-encoding, meant for longer projects
pro quality - two-pass VBR-encoding, meant to squeeze even more on a disk...
'quality' itself is defined by bit-rate - no manual setting optional in iDVD, simply determined by length:
≤60min = ~9 mbps = maximum bitrate allowed by standard
60-90min = ~6 mbps = acceptable quality
90-120min = ~4 mbps = who wants to watch two hours of shaky homevideo anyhow? 😉
so, for 'best' in iDVD, stay under 60min, set 'best performance'
for worse, squeeze as much as possible onto a single disk.....