I'd wait. I am in the same boat although my 2010 is being problematic on a daily basis. Holding out as long as I can (hopefully until new machines are announced.) I am vacillating between a 27" gen 2 retina or a souped up Mac Mini.
I played around with your exact spec'd 27" retina iMac on Tuesday (it had the X designation video card), super fast. I forgot how much space the SDD portion of the fusion drive has though, I am sure other folks who own could tell you how much space the SDD has.