I'm getting a Mac mini this month, and I've been scouring forums to figure out what I need too
The average user will indeed be happy with a base i5. If read/write speed and disk space becomes important, get a kit for a second drive. A photographer will most likely be satisfied with a great, huge external USB3[1] drive if internal space becomes an issue. Few average Mac users use more space than them, I think
In my case I've figured out I need an i7. I do a lot of CPU/RAM-intensive things: Programming, audio work, benchmarksturbation and more. Razor is one hell of a CPU-hungry instrument! Disk speed hasn't been an issue for pure wave-based audio (handling over 100 tracks at the same time just fine with my old C2D laptop and a 7200RPM drive), but the soft synths and effects need extra cores.
My current setup is a unibody MacBook with a 750GB drive + external 2TB where backups, installers and the iTunes library lives. Internal space goes to development tools, Logic, Ableton Live, Komplete and a ton of additional instruments and individual samples. If I wasn't doing anything audio-related at all I'm sure I'd be happy with an i5, even for Xcode.
[1]Side-note about USB3 externals: I haven't found any that claim to be using anything more than SATA connectors. No SATA2 or 3! A possible reason to upgrade internal drives if you move *a lot* of data every day.
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IMO, the GPU is the bottleneck of this little sweet machine. In my situation, having an i7 is pretty much useless as playing games in HD4000 is not that "good enough". Sure it still plays some new games with very low settings, but in the end it will not please you that much. Most of the time that i7 will not *literally* do what it is capable of.
And an i7 isn't really doing much extra even if you DO have a proper GPU
In my quest to build a perfect gaming PC I've decided an i5-3470 (not even K) is plenty of power for gaming. There also isn't a huge gap between mobile and desktop processors in the i7 range anymore. i5s have a larger gap, but still not too shabby. If only the mini wasn't quite as mini, and could fit a discrete GPU, it would be a pretty nice gaming box.