Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Software is going to make a big difference. I think I would trust a nMP under heavy load on a consistent basis with a lot more confidence then an iMac. We're talking mobile GPU vs. matching workstation GPU's with the same VRAM (total).

The way I look at it is…if you buy the base Quad Core nMP, you're getting similar CPU performance to the maxed out iMac, with workstation GPU's, the same storage speeds for the most part, a lack of all-in-one design which some don't want, and a lot more ports for output…without spending too much more. You do get a nice monitor with the iMac but of course it's attached.

The D300 from what I've read is basically the W7000 with half as much memory and another factor which escapes me at the moment.

W7000
http://www.gpuzoo.com/GPU-AMD/FirePro_W7000.html

780m
http://www.notebookcheck.net/NVIDIA-GeForce-GTX-780M.88993.0.html

I think you'd find the speed fairly comparable overall. This is why I bring up the form factor element to the discussion. They're radically different machines in design. Even if their scores were really close or exactly even, I bet if I had my 24" Dell monitor hooked up to the Mac Pro and the 27" iMac ran the same renders, I might edge out the iMac due to the GPU not being as taxed by my screen size. Less pixels to keep pumping.

In real world use I'de like to think the nMP would give you better overall performance, especially on a consistent heavy load type basis…but only having numbers to look at and basic logic to use…I can't be sure.
 
Workstation GPUs are not as good as running games as gaming cards, and gaming cards aren't as good as workstation cards running workstation tasks.

If you're a gaming type, go for the iMac.

If you're one of those that do stuff like Maya, go for the Mac Pro.
 
That depends on what you are using the system for. If you're talking for games, you want the 780M. Other than that you would need to look at each app you want to run the is GPU dependent.

In eed the GPU that gives me the best 3D rendering performance. I want to use it for Microsoft Flight Simulator X. That is not à game but more simulation. Therefore I dont need 60 fps. 30 fps is enough
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.