Intel all the way - much better for the server enviroment, less heat, less power usage, less of a risk reliability wise and much much more r+d into their server processors (not just taped out asap like AMD)
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If it were Opterons, I'd say go for Opterons over Xeons. I've seen very busy sites using Athlon MP and they do fine, just as good as any Intel processor. AMD processors definately has more bang for the buck.
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For pure speed, I'd choose Opterons... but presuming all other components are (for all practical purposes) the same and none of the hosts in question overloads their servers, there will be very little difference with ANY CPU when hosting typical web sites. Raw speed is great, but the truth is: a simple html site handling less than a few thousand requests an hour is not going to be noticeably faster to the end user on a quad Opteron than it would on a single P3.
If you are legitimately that concerned about CPU speed, you should probably be in the market for a dedicated server, not shared hosting.