I speak of the iMac from personal experience. I have a 2011 27" 3.4 i7 with an SSD and 2tb HD plus a Pegasus R6 for video, photos, and system backup.
That iMac runs hot. All the time. Sure, I understand the case itself is a heat sink, but just recently the 2gb ATI 6970m on my iMac just went kaplunkt. Good thing is there was an issue and a bulletin from Apple regarding failure of this particular configuration. When I talked to the tech at the Apple store, it was very clear to me that heat dissipation was an issue faced by all iMacs. I hope most people never see the screen of death which I faced, but the high temperature, and my personal experience, make me lean to MacPros as opposed to iMacs as my workhorse.
I was without my iMac for about a week, and let me tell you, it was not pleasant. Fortunately, my 2008 3,1 was humming along and I was able to continue working without any fuss.
So in short, maybe the 2008 3,1 MP is older hardware (it has 4 micron m960 1tb SSD in RAID 0 and it is maxed out with memory plus a modded GTX Titan), but this beast has been on for the better part of 5 years and it has never gone down except for upgrades.
Uptime, priceless. Downtime, unacceptable. The turtle always wins the race because it never rests.
And the new trash can MacPro is built with better components than any iMac currently in production. MacPros were designed to be on and functional all the time. I don't think I could confidently say this about any iMacs.
So when the nMP comes out, I will be one of the first ones to get a fully loaded machine. I plan to keep it running for the next five years until it fully depreciates.