Well, the best advice anybody can possibly give you upfront is:
Go into an Apple Store and try them both out.
Then, ask yourself if you want the version with wifi and cellular data or the one with just wifi. If you plan to use the tablet on the go, will you be able to use your smartphone as a wireless hotspot? If you plan to get a contract from a wireless carrier it might be worth checking if they offer any bundles.
The mini is much smaller and lighter than the retina version of the iPad. The retina version on the other hand, is the fastest iOS device out there at the moment and features an outstanding display.
Both devices can do pretty much the same thing. It comes down to personal preference I think. The mini is much more likely to be carried around all the time and has that Kindle-sort of size and weight that makes it a joy to pick up. The retina iPad will give a much nicer experience on long trips or other sessions, but it is the heavier and bigger device.