That depends on your normal workload. Most users would never notice the difference in processor speed, since most users rarely push the CPU to its limits. You'd see much more performance boost from increased RAM or a SSD.
Definitely save the money and put it towards an SSD. This single upgrade will prolong your Mini for many years. My brother has a 2006 MacBook Pro that I handed down to him a few years back. A few months ago, he wanted to upgrade to a new machine. I told him to wait until the new MacBooks came out and helped him install a 120GB Intel SSD. He's telling me that the MBP will easily last him another year or two before he really needs more power and has to upgrade.