It depends on when you need it. I have been very upset at how long it is taking for a new 5K standalone monitor to come out. It has to do with the move from Broadwell to Skylake chips. I read somewhere (MacRumors?) that the Skylake chips were getting closer.... That should mean a new Mac Pro in the coming year, one that would have the capacity to drive a 5K monitor.
Intel Skylake Processors: Up to 20% Performance Boost, 41% Faster Graphics and 30% Longer Battery Life - Mac Rumors
I had a conversation with my "Mac Business" guy at the local store. Almost always a better choice to talk to these folks. He started by suggesting I get the iMac 27, he said it would be just as good. But after some conversation, the effects of multiple GPU's, the increased expandability, we decided that in fact, the higher end workstation was going to be the one.
If you have minimal needs, I think either would work very well. However, the Mac Pro, especially next year's Mac Pro is going to be the choice for me..
Lenny,
I was under the impression that the new trash can mac pro and multiple gpus was designed more for video and 3D rendering, and that it wasn't going to be as much of a benefit for working in photoshop. I haven't talked to our apple store business guy since right after the cylinder mac pro came out, and have just been upgrading the 6-core 2010 cheese grater with more ram and SSDs, but the thing I still don't really understand is if the faster/dual gpus will make working with huge files faster (maybe that is where the load of VRAM is beneficial?), or is that still dependent on single threaded processor speed and tons of (fast) RAM.
The question I have for the next few months is how Metal for in El Cap is going to benefit future releases of photoshop and working with huge drum scans (you're the only other person I know here who works similarly to how I work).
When it was time to "upgrade" my home machine earlier this year I bought a 2009 mac pro, and put in more ram, a couple SSDs, and am about to go to the latest apple supported video card in the next few months. If I find that i really "need" the use faster ram, I can upgrade the firmware to use the 2010 model 1333 RAM. Aside from the video card difference, the upgrade that would make it more in line with the 2013 Mac Pro would be putting in a couple PCI SSDs. The real time suck for me is reading and writing 4-8 GB layered working scans.
The only thing I really pay much attention to for Apple updates is listening to Accidental Tech Podcast. From what I understand, the skylake chips won't come to the mac pro, and that it wont be upgraded until there is a new set of Xeon chips, which might be behind schedule. It doesn't sound like the new ones will be much faster in terms of clock speed, but will have faster RAM.
iMac: Personally I would never get an imac. Those still use laptop/consumer components and can't be upgraded. The Mac Pro uses server grade chip sets and much better RAM. The clock speed might be slightly slower, but a Mac Pro would be more stable and less prone to memory errors/kernel panics and will last a lot longer and hold its value longer too.
I know that the stuff I use them for are kind of special case scenarios, but in terms of expandability and long term value there it is hard to beat a 2009-2012 mac pro and incremental upgrades. The point is that there are much more affordable options than the tricked out new hotness if you aren't doing the kind of video and 3D work it is designed for.