Looking at 6-core Xeon to dual 6-core Xeon architectures, it can quickly get confusing. In simple terms, I've found that if a PC can run the latest games, it can usually anything that I throw at it. This is especially true for some power users like me, who don't really play any games on their computers.When comparing the top of the line Mac Pro, with a 2×6-core Intel Xeon Westmere processor, it's surprising that alternatives from Dell, HP, and other mainline manufacturers are almost the same price. Having the choice between these workhorses, I'd probably choose a Mac Pro, hands down.
The reason is if you add up the cost of the components separately, you'll quickly end up with a similar price. As an example, just the cost of processors (two in the case of the 2×6 twelve-core workstation) and a motherboard with similar performance, you'll already end up paying $2,600, and that's without RAM, a case, a graphics card, and other minimum components.
A slightly cheaper alternative would include buying a gaming rig to use as a workstation. An interesting workstation to choose would include the Alienware Area-51 ALX. The highest option comes with a 6-core Xeon Westmere processor, but it has the benefit of coming in a cool liquid-cooled case, which is really interesting.
Ultimately, if choosing between a Mac or PC workstation, I'd probably choose a 2×6-core Mac Pro with minimal options. Once I receive it, I'd max the RAM out by using non-Apple chips. Then, I'd pop in Western Digital hardrives, probably 2×2TB. The trick is deciding what kind of workstation you need. The only thing that's stopping me is the price. I've found that if I purchase a high-end workstation, it can easily last 2-4 years without any problems. The added benefit is that these workhorses will have no problems running multiple monitors, which is always nice.
[images via Thermaltake, Apple, and HP]