Never owned the 5Dc, but played with a used one before I bought my 5dii. Low light is definitely better on the 5dii. I found ISO 3200 on the 5dii to be about the same as 1600 on the 5dc. The 5dii also has live view, a better rear screen, sensor cleaning and MFA. IQ differences at lower ISOs are minimal and some people prefer the 5Dc's "look". They are both great cameras so long as high perf AF isn't important to you.
If I owned the 5dc and wanted more modern "accoutrements", then I'd go for the 6d, or if I had the money, the 5diii.
If you do mostly portraits, then keeping the 5dc and getting an 85L might make more sense.
+1. OP, I'm about to make the jump to FF too and had long been eyeing up a 5DII, the 6D wasn't even on my radar, but after reading a bunch of reviews it ought to be on everyone's radar. The improvements are fairly drastic in certain areas, to me it's a huge upgrade to the 5DII and because of that I'll be getting one really soon 





