I am not sure what your usage habits are, but I do a lot of video editing with motion graphics on my PC rig. Only rarely have I actually had Windows/Adobe tell me that I was running low on RAM due to intense usage (scrubbing and quick rendering, I would regain more free RAM if i just waited a few seconds).
If I am just doing normal things like games, heavy-web browsing (40+ chrome tabs), music, multiple (non-editing) programs open. I never even get close to blowing the 8GB cap.
I have yet to see anyone who isn't a video editor/graphics person max out 8GB of RAM on normal usage.
Most people usually opt for 16GB, just because 'they can', but for the average user, it's a waste. The placebo effect of 'having' that 16GB, when their usage habits don't ever get to that level is quite common nowadays. I'd use the extra $40 you get from saving the RAM and putting it towards an SSD.
And since this is the Mac mini forum, I am assuming this mac mini isn't going to be for heavy video editing.
Now unless you're going the server option, then yeah, you'll probably want the RAM, but not sure why you'd choose dual-core then.