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what Iowalynn said. my 2012 mac pro fully loaded 12 cores sapphire mac edition graphic card PCIe ssd blade with 64 gb of ram and 4 2tb hard drives beats the new iMac 2015 5k.
 
it's not just about the fastest scores and peak single threaded performance. The Mac Pro excels at being a fat pipe for data. It has 6 TB2 and 4 USB3.0 connectors as well as an HDMI port so you can connect your display.
I run Logic pro with and also do a lot of Premiere and After Effects stuff. I have a couple UAD DSP processors connected via TB2, a pair of MOTU soundcards connected via TB to firewire adapters and a couple disk arrays connected via TB2.
My Mac pro never breaks a sweat chucking data around.
I've been doing fine with only 16gb of ram. But I don not often use large sample libraries as a lot of people do.

As an example of how adept the machine is, I often have a Linux VM running on Virtualbox, while I am editing video or working on a song. I have rarely had to close any of my programs due to performance hogging. I'll leave Premiere, Logic and Virtualbox open all the time. The exception being After Effects, I close everything for that hog.

As far as upgrades. My policy has been to lock down my DAW for a couple years at a time. It is not a good idea to upgrade your DAW while you have projects still pending. This includes hardware as well as software.

I'd also add that I switched to Mac for work reasons but have since found that production on a Mac is smoother, more stable and less headaches than working on a PC. The 'Apple Tax' is often mentioned, but conversely when I worked on Sonar and Cubase I spent a LOT of time troubleshooting audio driver issues and stability problems. I'll pay a little extra not to sweat that.
 
I was in a similar position when the 5K iMac first came out and really wanted to justify getting a nMP. But I wasn't able to. If you are just comparing a base nMP, I would go for a spec'ed out iMac. Unless you need things like 6 TB2 ports, upgraded graphics cards, or require bragging about your computer.

I purchased a Late 2014 27" Retina iMac, 4GHz i7, 24 GB RAM, 512 SSD right when they came out and have no complaints. My use case is a little different, though I often have Linux and Windows VMs running at the same time as a dozen plus other application including things such as Lightroom, Xcode, Android Studio, etc. and I haven't had any problems. And I do use this machine for my livelihood.

Though I would definitely get a refurb if getting a nMP.
 
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