Than antenna and transmitter between all the inReaches are the same per Garmin.
The only important differences between the mini and the Explorer+ is battery life (the Explorer's is significantly better) and frequency of message checks. The mini automatically checks messages and sends tracking points every hour and this cannot be chaneged, though manual checks can be forced (the reason is battery preservation). The Explorer defalts to checking every 10 minutes, and this can be changed to more or less frequently.
Most people are fine with the mini. If you plan multi week expositions, need near real time messaging, or need frequent tracking points, then the Explorer is what you want. Most people never notice the delay with the mini as it also checks 10 minutes after a message is sent, so if someone replies quickly you will get it at this 10 minutes check. However, if they take 11 minutes to reply you will not see it until the hourly check unless you force a manual check before then or send another message.
Glacer travel is the only time I can think of where I would defenatively want tracking points sent more frequently than hourly (if you set the mini to 10 minutes tracking intervals it just sends 6 tracking points at the 1 hour mark, it does not send 1 tracking point every 10 minutes).
In short, you probably want the mini.