The MacPro is the boy. I'm pretty happy with the performance of my now 5 year old tower, especially since going to a true 64bit OS, and doing bits and bobs in 64 bit versions of Premiere and After-Effects, however, it isn't going to last forever, I'm already onto my third graphics card (known issue with the early 2008 mac pro, now fitted the radeon 1gb option)
But as its a cap ex, I won't get money for a new tower before march, and from march you can't buy a mac pro in the EU (seems to be a temporary measure) if the new tower is absolutely brilliant and a major step up this time then I might spec that as my next machine, but as I'm heading in a less mac proprietary software direction (premiere instead of FCP) it becomes a question of how much do i really really like the mac OS. The answer is, a lot. An awful lot. But then the difference in cap ex, is also a lot. An awful lot. Not that its my money, but my hd-25's are on their last legs.. as is the eng tripod.. we're using a domestic samsung LCD for client playback since the broadcast monitor died of obselete major organ failure...
If you are going to cut on FCPX then a macpro tower, when they upgrade, if they upgrade, running the OS off os an SSD fitted in the 2nd optical bay, and running two striped software raids in the 4 bays, one capture, one render, is absolutely the way to go.
If you aren't going to be cutting on FCPX (and I'm not) then a macpro suddenly looks like an unnecessary expense. Especially compared to a PC with mercury engine card.