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.