This solely depends on how much you want to interact with the program and adjust your own settings.
If you want to have all the freedom in the world to make your own custom preset and have all the fun and cool Iris features, pick the regular Iris – it comes with a 7-day free trial and you can buy it after that (see How to buy Iris section).
If you don’t really want to hassle much and just want a couple of pre-made modes install Iris Mini – it’s a minimalistic version of Iris.
Iris Micro is the super small, super-fast, command-line version of Iris, it’s practically invisible.
Iris Vision is a software for the color blind.
For more information about each model refer to What features does Iris have section.
This is answer also to:
Which is the best model for me
What is the difference between models