Earlier quoted context omitted.
AMD makes 16 core Opterons (codename Interlagos), however they're slower than the latest 8 core Xeons. http://en.wikipedia.org/wiki/Opteron#Models
Slower per-core, certainly. But for some workloads more cores is worth it.
In this Anandtech review, the 16 core Opteron trades blows with a six core Xeon (clocked at 2.3ghz vs 2.66ghz):
http://www.anandtech.com/show/5058/amds-opteron-interlagos-6...
The 8 core DP Xeons weren't out yet when that review took place, but the Sandy Bridge chip was a big improvement over Westmere, in addition to the extra cores.