Trying to decide between the 66i & the InReach Explorer+ .... main differences?

30 second send (via Iridium) intervals are not possible with a personal account with either device. The minimum for the highest plan is 2 minutes. I don't know about a commercial account. 

Logged points (which do not show up anywhere except the device until after a sync operation - wired or OTA) can be recorded down to 1 second on both devices - although anything 30 seconds or less eats battery on both. Note that the control of logged point intervals is substantially different on the 66i than on the earlier iR devices.

With regard to 66i battery life, I've done a little bit of testing with what it calls "expedition mode". Although I can't confirm the life Garmin claims (220 hours or something that?), it does yield considerably better battery life. Expedition mode disables the screen, BT and ANT+ radios. It does not limit the send interval, although I am sure you will get better life with longer intervals. (Can't remember, but I think the 200+ hour claim was with 30 minute send interval?). The logged points (the recorded "activity" in 66i terms) are saved at considerably longer intervals - but the resulting track is still quite usable. You can't actively use the device for navigation because the screen is off. However, you can take the device out of expedition mode, use it as normal for a while, then put it back in expedition mode. Overall, I'm pretty pleased with the result. But I would want a lot more real-world testing before I headed for the back country without a solar charger or other means of recharging the device.

The major difference between the devices is that the 66i is a full handheld GPS system. It includes all the bells and whistles of the GPSMAP 66st. This includes fully routable (road and trail, as well as your own routes and tracks) cartography in Garmin format (as opposed to the legacy DeLorme format used by the Explorer+), the ability to use Garmin "custom" maps, and the ability to use third-party maps such as OSM and those from gpsfiledepot. It includes features like paperless geocaching, the ability to pair with ANT+ sensor devices, and so on. The recorded activities integrate with Garmin Connect via the Garmin Connect Mobile app. The newer Explore app replaces the legacy Earthmate app. Some of these differentiators won't matter to people use the device only for back country navigation and safety. So YMMV.