In my opinion, NEITHER of the options you have chosen.
The revB MBA 2.0, is the best way to go. It is an absolutely amazing computer, and it is a real fully functioning MAC. The MBA revA was a really slow, underperforming, and unMac like.
The iMac is really due for a revision or at least an upgrade. The MBA revA is not worth half of that. And the MacMini is over 18 months old.
Most people don't realize just how great the performance differences between the old MBA and the MBA 2.0. The new MBA is fully capable of driving a 30" Apple Cinema Display.
Old vs New Comparison
PATA Drive vs SATA-II (huge speed difference, the MBA 2 has a much faster HDD than the original SSD)
Intel Graphics vs Nvidia GPU (the MBA v2 has four times the graphics performance - connect a 30" display, too)
Overheating chip vs A real PENRYN 45 NM CPU (the MBA v2 has the same type of CPU as a MBP or MB + it has 6 MB L2 Cache)
667MHz Memory vs 1066 MHz Memory (that's a noticeable difference)
80 or 64 GB vs 120 or 128 GB (drive space is double with an SSD in MBA 2)
Core Shutdown and Overheating vs One Powerful Fully charged MAC! (the MBA v2 is absolutely amazing).
So, my advice is to consider buying a new MBA revB 2.0 and an external display. I would really suggest not getting the SSD in the revB and instead spend the extra money on a nice 24" LED Apple Cinema Display.
Or, if you don't want a MacBook Air revB, 2.0, and you absolutely are stuck on your decision, WAIT. Wait it out to get an updated MacMini or iMac. I think that both are coming out between now and the beginining of March (based on rumors and the big wait).