There are three base configuration options for the Mac mini. The first two base models have a few significant differences.
Mac mini 1.4GHz Dual-core vs. Mac mini 2.6GHz Dual-core
1) 1.4GHz Dual-core Intel Core i5 vs. 2.6GHz Dual-core Intel Core i5
2) 500GB vs. 1TB
3) 4GB vs. 8GB
4) Intel HD Graphics 5000 vs. Intel Iris Graphics
5) $499 MSRP vs. $699 MSRP
With only a $200 price differential between these models, the higher-end option is a great deal for a faster processor, additional hard drive space, twice as much RAM, and a better graphics card.
The mid-range and high-end model only have a couple differences.
Mac mini 2.6GHz Dual-core vs. Mac mini 2.8GHz Dual-core
1) 2.6GHz Dual-core Intel Core i5 vs. 2.8GHz Dual-core Intel Core i5
2) 1TB hard drive vs. 1TB Fusion Drive
3) $699 MSRP vs. $999 MSRP
For an additional $300, a slightly faster processor and the upgrade from a standard hard drive to a Fusion Drive doesn't seem like enough to justify the upgrade.