The max transmit power does not really matter that much. You shouldn't go over 14 dBm anyway.
The antennas used are the same.
The receive sensitivities do seem a little different where the MR84 seems to have better sensitivities at the lower MCS's but a little worse at the high MCS's (depends on the radio, it seems to differ quite a bit).
However being able to do explicit beamforming with 4 radio's instead of 2 will improve range for multi radio clients.