Note: My response can only lend to support for you wanting to use Opteron instead of Xeon. I have no experience with Debian, so I will not comment on it.
For the longest time, all of my new servers would be configured in some way using Dual Intel Xeon CPU's. The most recent of which was a dual Xeon 3.2 Ghz (64-bit).
Those days have long gone as I now choose AMD Opteron due to its sheer performance.
My latest machine, a Dual AMD Opteron 270, lays waste to that Dual Xeon I mentioned above. The dual Xeon has 4.55% disk saturation and processes about the same amount of data as this new Opteron; however, this new Opteron has about 20% disk saturation. The load is substantially less on the Opteron.
Another time I upgraded from Dual Xeon (64-bit) to Dual Opteron 250. Yet again, another amazing boost in overall performance.
In my opinion, AMD Opteron CPU's are better than Intel Xeon.