I have decided I want a desktop computer, and I'm pretty sure it will be a Mac mini
Good on you for looking to a desk top. Portable computers don't really do it for me. It feels good to leave my Mini behind on my desk when I go off to do something else.
That being said I am fortunate to have had a fairly settled existence for the past decade. However, with a more itinerant phase looking likely in the next little while, I could land up stumping up for an Air..... with misgivings.
will the mini last me very long before it too needs to be retired? I am leaning towards the i5 but should I go for the i7?
My first Mini (and my first computer) was the first, which I used from 2005 until 2009. It had its shortcomings, and I slightly cynically felt the original Mini was a way for Apple to use up its stock of laptop parts on its way to making the move to Intel chips. I chucked it when the HDD failed. Fortunately I had most of my important data backed up so it was no big deal.
I replaced the original with my current Mini, the early 2009 base model, which is probably the first to be a fully fledged Mac, rather than the under performing baby of the family. With only 1 GB of RAM it became quite sluggish when handling updated software, probably designed with more in mind.
The addition of another 4 GB of RAM, and installing Mountain Lion last year gave it a new lease of life. It is more than satisfactory for my relatively simple needs, and should remain so for a few more years.
In short, if a laptop has served you needs thus far, chances are a new Mini (maybe with a "mid-life tune-up") will do likewise for a good few years.