If you don't like overclocking, the Xeons are even better for you, as they don't give you that choice of luxury :- ).
Anyway, Opterons are out. They haven't been updated for very long time, and AMD had withdrawn from the high-end market since, this includes server platform.
If you mean the last high-performance architecture they had, the 6xxx range, than 64 cores equalled up to 30 in Cinebench score. Similar to Xeon range but with following drawbacks:
4xCPUs draws 4x140W. Compare it to 2xXeons at 115-130W, that's twice power.
They require server based OS system to run. Not a big problem, but inconvenience for workstation.
The architecture isn't so universal as the Xeon range. Even though syntentic tests shows potency, this didn't prove in practice with many renderers. Vray for particular, had big trouble feeding the cores and recorded subpar performance. You
can search Vlado's answer to this on Chaos forum. Advice:Stay away.
Extremely poor single-threaded performance. We're talking, half of that per core to SandyBridge architecture in Intel, at clock up to 2.7Ghz. That's absolutely useless for any workstation task. It's like buying Celeron.
tl,dl: It looks nice on first sight, but if something looks too good to be true, that it probably isn't. Opteron range isn't.
You already bought the worst AMD ( I really like the company...but, you basically have to ignore their cpu range recent years) cpu out there, don't repeat the mistake.