from a report written on 3/29/16
LG G3 Marshmallow Release Details on AT&T, T-Mobile, Sprint and U.S. Cellular - Neurogadget
AT&T
AT&T has not been very busy in the Marshmallow updates, with only a handful of devices running on this carrier currently having received the update. The carrier has not mentioned a word on when the LG G3 Marshmallow update will show up. The last time this device received an update was back in 2015, which was basically an update to Android Lollipop.
Perhaps AT&T believes in the adage "If it ain't broke, don't fix it".
Here's a comparison of Marshmallow and Lollipop:
https://www.androidpit.com/android-m-vs-android-lollipop-comparison
Some of the highlights:
App permissions. Allows one to control individual permissions of apps.
Good for those who are smarter than the app developer.
Battery life: Doze. Claims to "double" battery life.
Very skeptical. The three biggest battery drains are (1) screen on time, (2) distance to nearest tower, (3) GPS.
How will Doze help with 1 and 2?
Google Now on Tap. Too intrusive AFAIC
Fingerprint sensor: LG G3 ain't got one.
Android Pay: works on Lollipop with pattern or PIN, don't need fingerprint.
Only significant advantage I see in Marshmallow over Lollipop is being able to use SD card as internal memory. However, it may require a re-formatted SD card, may slow down the operation, may not work correctly on g3.
Not anxious to get marshmallow at all.