The Opterons are the clear leaders from a power perspective. Shanghai uses approximately 12% less power than Barcelona. Intel uses 27% to 46% more power than Shanghai, depending on the DIMM configuration. At all load points, Shanghai is the clear winner. For the first four load points Shanghai is again ~12% more efficient than Barcelona, and as much as 28% or 47% more efficient than the Intel systems depending upon the DIMM configuration.

http://it.anandtech.com/IT/showdoc.aspx?i=3456&p=1