Mainly it is the build area. The Taz 5 is a bit less “friendly” for lack of a better word. Out of the box it is lacking the auto leveling bed but Auto bed compensation can be added. The extruders are virtually identical though.
If you just have/want one 3D printer, the Mini is nice. Low-maintenance.
A Taz is nice to have for the larger build area, but also for its more open construction. Great platform for experimental tools (eg. lasers) and various other upgrades.