Well, it all depends...(doesn't it always?)
The top end iMac is a really well speced machine equal to or perhaps better than the low end new MacPro 2013. But...will 32GBs be enough (if you are working with 36MP cameras)? The new MacPro can go to 64GBs (and don't dismiss this as available ram is a major Photoshop performance factor).
The internal SSD is gonna beet the pants of the iMac. External connections via ThunderBolt 2 (TB2) will be about the same. But the vid card (GPU) is much stronger in the McPro 2013.
Yes, you can pack the top end iMac to be a real powerful Photoshop/Lightroom machine...but the MacPro 2013 can beat the iMac if you go with the 6 or 8 core machine (the 12 core isn't really a Photoshop.Lightroom machine).
So, can you get a good digital photography machine with the top end iMac? Absolutely! Can you set up the new MacPro 2013 to be the best in breed? Yep...it all depends on what you are willing to spend to get what you think you need.
1. It's hard to imagine it wouldn't be. With the way PS uses scratch disks it all comes down to how fast you perform the functions you use. 4gb will "work", 32gb is downright luxurious in most cases, 64gb.. you'd have to be working a ton of complex images a day to get much value from that.
2. Having PCIe SSD's in use and SATA3 SSD's on the same machine, on different machines, etc.. the difference is actually hard to feel. We'll have people who have never had a SSD, or only had cheaper slow ones, who will be impressed with the PCIe's performance. But anyone with a fast SSD, overall, will give it a "shrug.."
3. Curious what you mean by this? Is 12 cores just overkill and more appropriate for video applications, or is LR somehow limited on the cores? We know LR is CPU intensive so I'm curious.
4. Agreed, either, properly spec'd, will make a very good machine for still imagery. I think the current trend is people are buying a lot more hardware than they will really use.. If someone isn't working extensively with video, or isn't time crunched to process a high number of images a day, and otherwise is happy to work at a relaxed pace.. then they should error on the side of a good solid quad core machine with up to 32gb and not everyone needs that much. Many of us were/are recently using i7-920/930/950 machines with 8-12gb of RAM to process high MP images with relative ease.. I'd could process 400+ images a day with that machine without much effort. Few were complex, but still. With RAM being so cheap these day.. 32gb is a luxury most would be well advised to go to..