I'm kinda in the same boat. I recently sold my near enough maxed out 5,1. I just didn't need the power anymore as my work was finished. Since selling it, I've been left with a hole which I don't know how to fill, or even if it can be.
I've thought about getting a macbook, no - battery cannot be replaced and ram can't be upgraded. Imac - nice but Mac Pro territory money. Mac mini, the original reason I moved to macs - very tempting until I saw the new version had soldered memory...
So I've settle with getting a Mac pro, but the 6,1. It's not ordered yet, a little scared to if I'm honest, as I'm not 100% sure yet, and I'm fearing an upgrade to xeon v3 fairly soon. Around a day after I click 'buy'
So the Mac pro 7,1... I realised earlier that this is not the replacement I'm really waiting for anyway, it will be the 8,1 (probably) or whichever has thunderbolt 3. As this isn't out in desktop until 2016 now, and the xeon counterparts are after, it's too long to wait. What does v3 get me over v2? Ddr4... Which will most likely be slower and more expensive for a short while compared to ddr3. And not a lot else, extra instruction set I won't worry about, and a bit faster and potentially more cores.
Upshot is, there are now some cracking deals on the 6,1. I am pretty sure I'm going for the base model of one of these now. Even knowing they will probably be replaced soon. Just slightly apprehensive about the d300's driving 4k...
Nox