This is not a complete answer, but as you've been waiting some time, here is the little that I know:
LXQT Includes a graphical user interface using QT. This takes up some space, so I believe the images are bigger.
IOT No GUI, so smaller.
reference Molloy, "Exploring Beaglebone"
Flasher Automatically reflashes the eMMC when you put the SD card in. This is too dangerous for someone absent minded like me to have around, especially since all SD cards look the same. The automatic flashing can be defused but personally I would always start with a non flasher and use Robert Nelson's scripts to flash the eMMC if desired.
Best of luck,
Alan