I'm not an REI employee but...

The larger units are self contained GPS mapping devices with the InReach satellite messaging feature.   These units are probably more rugged than a phone.  They are probably easier to used to send or receive satellite messages because you do not need to pair them with a phone and they have larger screens and better input methods than the mini.  Having a standalone unit provides redundancy.

The mini, while it can give you a GPS position has no mapping built in and is really primarily for satellite messaging.  You can use it with your phone to do mapping via the Garmin App by most smart phones have GPS mapping available which works with other mapping applications.

The mini is much lighter and if you have a smart phone is probably more than adequate for most people but you do have a bit more setup and you have to remember to install apps and maps on your phone to get the same function and have a way to your phone safe while hiking.