Star Alliance is the larger of the alliances, however it is common that One World has better internal linkages between airlines.
Lot of internal problems and errors when flying between Alliance carriers, however with one World it seem to be more seamless. This is what my customers have told me, I have found this an overwhelming scenario.
In essence , I guess it depends on you itinerary to find out who would appear to be the better of Alliance.
However from an Australian perspective one world is better since there is no Star Alliance carrier in Australia.
In general the southern hemisphere is better for One World whilst the Northern Hemisphere is better for Star Alliance. This is due to the following:
Star Alliance pretty poor in Africa, One world has Comair
No Star Alliance carrier in Australia
One World - connections to South America ex Australia, good feeder traffic through AA
and then there is Lan Chile and Lan Peru.
No mainland Continental Europe carrier in Europe as yet for One World**
Asian Connections pretty poor in Asia for One World
** Swiss to join around March June 2004