rMBP - Do I need the dGPU?

Do your programs have graphics acceleration through OpenCL or CUDA? That could make all the difference.

CUDA = nvidia only
OpenCL = nvidia or iris pro
none = GPU is not going to make a difference for rendering

 

I think that if you're using CS3, it won't matter. I believe that CS3 has minimal if any GPU utilization, so it's pretty much entirely CPU bound.

If you choose to upgrade to CS6 or CC, then that's where Iris Pro really shines from what I've read.

As far as your dual monitor heat concerns, I believe that any modern machine should do fine without heating up too much. My 2010 MBP lives about half of its life at my desk (using its dGPU), and I can't hear the fans until I start pushing the machine.

 

If you are going to be using Creative Cloud and can afford the extra expense then get the dGPU the CUDA acceleration will save you time in the long run and if your looking at using the machine for the next 6 years might as well go all out.

 

The dGPU doesn't have its own fan. Both fans always cool both chips. There are two heatpipes (one for each fan cooler) going over both chips.

I used to have a 2010 MBP before and since yesterday the new Retina with the dGPU. So I can give some info on the heat issue. The performance issue should be straight forward. You need a dGPU only really for gaming and in battery life the Iris Pro should be much better in low to medium load scenarios.

On my old MBP when the 23" 1080p external was attached even medium load pushed the fans up to 3000 rpm and more. Only very low load kept it at 2000rpm.
The new one does much better though. I stays at the lowest rpm even when I use load that used to qualify for medium.
I.e. VLC post processing. You can try that too if you have some HD videos.
Open on the external screen a 720p video and activate in the video effects CMD+E sharpening. Very useful for better upscaling such material. On my old notebook that used to push fans quite a bit up to around 3000-3500rpm. With 1080p material it dropped some frames and load was about 100% on one core and pushed fans to 4000 and higher.
On the new MBP only 1080p material pushes fans out a little and just to about 2600rpm.

If you really push it though with full load, the new MBP heats up faster. Those newer turbo modes are very aggressive and it heats up quite fast. The external screen seems to pose much less of a problem now for heat alone then my old 330M. With my old there was a huge difference with integrate only. Basically I needed 80% of full load to change anything about the fans then on integrated while with the dGPU 30% CPU load in 2D environment did the same.
I don't have CS3 nor any other serious photo editing software. I only use GIMP rarely. Somebody else can test it.
I did a little handbrake testing. Just plugging in an external screen and running the 750M make absolutely no difference whatsoever under full cpu load. Performance is the same. CPU turbos the same (50W for a bit then it goes lower). Even under Iris Pro it heats up extremely fast and hits about 99C within a couple seconds. (One core peaked at 100C). Fans go to full speed and stay there with CPU temps between 94-98C.
750M active and external screen attached shows the exact same thing. There is no GPU load whatsoever in the test but in reality either you app uses the GPU then the CPU isn't under full load or it doesn't. If the dGPU is active also seems to change nothing about the power budget the CPU cores take for themselves. Not having the Iris Pro active changes nil.
For the most part it is a wash on those new machines. Iris Pro only will be better for people that do lots of presentations or need better battery life in Windows but when battery life isn't too important the heat alone seems to make little difference.
Battery life is really not that great though and it would be nice to have QuickSync access in Windows. I need it too much for gaming so I got the dGPU and at the moment battery life isn't such an issue. I am fairly happy with the new model. It isn't any less noisy though than the old. At full fan speed the fans are almost louder just a bit lower pitched and more wooshing without the added high pichted engine whine. The old fans were fairly good too, these are just slightly better in sound characteristic but they seem a little louder on full 6k rpm. It may just be the thinner chassis and how the sound travels but subjectively it isn't an upgrade in noise.
Just browsing doing and typing this after Handbrake shut down it very quickly goes back to silence and it sits now at 50-53C for the CPU with the external screen attached and the 750M active. Low medium load as mentioned above with some advanced vlc movie consumption puts it at silent 65C.