Just some quick thoughts:
The iMac has a Fusion Drive which is actually a slow 5400RPM hard disk paired with a 128gb SSD. Personally, I don't like these - they were sort of a stop-gap solution to get a little better performance out of a slow hard disk when SSD's were very expensive. How well it works will depend on your usage pattern, but IMO there is little doubt that a SSD is far superior. And the SSD's on the new Mini are REALLY fast, so if 512gb is enough storage space then that's a big win for the Mini. If not, you could get the 1tb SSD for the Mini or plug in an external SSD for some additional space.