I have three options with the current 21.5 Mid-2010 iMac which doesn't support Mojave... Continue to wait for the inevitable space grey iMac that comes with an SSD drive (no reason not to do this if they just did it to the Mac Mini)... Buy the Mac Mini and sacrifice amazing display and the ability to some gaming... Buy NUC with Windows 10 and save $1-1.5k (over what a base 27" iMac will cost).
I might choose one and wait... The one thing I liked about my iMac was that even after eight years it still works amazingly fast. Granted, this is largely due to the fact that I did the manual SSD upgrade. The Mac Mini looks great but having experienced the Early-2009 Mac Mini and how fast it became slow (even with a Crucial SSD inside), I don't see it lasting as long as the iMac. Plus I'd have to buy a FHD-WQHD screen as I'm certainly not going to buy a $+1k screen. Finally, the NUC... I bought one to my elderly mother and it works just as flawlessly in her use as her old Mac Mini did. However, for my multitasking purposes, OS X is just better than Windows 10 in terms of speed, ease of use and that premium, everything-works-and-then-some feeling.