Well, here's a real life example of making this decision.
The most demanding things that I'll do with this machine are use Final Cut Pro X to edit 4K footage (UHD 3840x2160) and Capture One to edit RAW photos.
Tomorrow morning, I'm going to one of the NY Apple stores to have a look at a mini up close and personal.
If I like what I see, and the store has a mini with an i7 processor, 8GB of RAM and 512GB of flash storage, I'll purchase it on the spot. I'll upgrade the RAM from 8GB to 32GB myself.
But from what I know right now, the store will in fact have a mini with an i5 processor, 8GB of RAM and 256GB of flash storage.
I just might buy it.
I keep computers for two or three years. I'm not sure that the i7 will make that much of a difference over that timeframe, and on the graphics processing side I intend to purchase an external GPU.
Also, I use a computer's internal flash storage strictly as a workspace. All data that is not being actively worked on is kept on external drives. With that workflow, I think that I'm OK with 256GB.
I'll listen to the advice of the Apple sales people, and make a decision. Right now, I don't know what it will be, but it could be an i5 mini.