THIS.
If you're going to be keeping the machine over the long haul and there's no reason you NEED the smaller form factor, there's really no reason to buy a "top end" 21 inch iMac right now.
Once you add the i7 and a fusion drive (or SSD) to the high end 21" iMac, you're already at $1899, which is within spitting distance of the baseline retina iMac at $1999, and not even that far off from the M290X model at $2299.
Going for either retina iMac gets you:
1. (Obviously) A beautiful, large 27" 5K retina screen that is a joy even once you've "gotten used to it."
2. A Waaaaaaaaaaaaaaaaaaaaaaaay faster GPU
Even the M290 in the baseline 5K iMac will absolutely destroy the Geforce 750M, while the M290X will make it look like an absolute dog.
3. A faster CPU for anything that can't utilise hyper threading (and in the case of the M290X model, most likely a faster CPU even in tasks that can), and the option (in the M290X model) for a CPU (the 4790K) that will be MUCH faster than the 21" top end 3.1Ghz CPU.
There's really just no reason to go for the 21" iMac right now unless you absolutely need the size, in which case even if you " can't wait" you should.