Having both, I can tell you I will never go dual cpu again unless it becomes the mainstream part of the market.
Single CPU systems (representing the majority of the market) benefit from broader compatibility testing, quicker turnaround on bios and software revisions hitting the web, and faster hardware components (ram, cpu) coming out sooner rather than later.
It's simply not worth the cost in comparison to saving the cash up front, by a single cpu system now and wait 6 months, and update the CPU/video/ram or some such in your system. Performance at time zero may be better for a duallie, but in the long run your total integrated experience for the same budget will be superior with the a single-cpu system combined with hardware refreshes.
My argument is meaningless if you are willing to spend the extra money for upgrading your duallie just as often, so I'm really talking about price versus performance.
I've gone single and dual Intel, single and dual AMD, and in both cases with the dual systems I found my investments depreciated a hell of a lot faster than the single cpu systems.
Just my opinion, but one based on experience.