The i7 Mini has more than just double the cores. It has double the accessible on chip L3 cache available than the i5 Mini which means less wait and lag states when it's doing intensive processing such as video editing, audio, etc.. It's also nice to have a quad core as more future programs are capable of utilizing more processor cores. If you are going to use the Mini for crunching away at processor intensive tasks such as video editing or if you want to future proof your Mini purchase then the i7 is recommended. For basic tasks such as web browsing and word processing the i5 with upgraded RAM (up to 16GB) should be good to go.
If you want to save some money go with the 2.3 Ghz i7 over the 2.6 Ghz i7 and upgrade the RAM to 16 GB when you can afford it. It's that not much more money than the i5. I have a 2011 i5 upgraded with 16 GB RAM and it works well but all of my 2012 Minis I bought for personal home use and business use are i7s.
