2mp gives a few extra degrees wider field of view over 3mp. Can be a + or - depending on one's needs.
I seem to have a sharper image and better live feed & recording with 2mp.
Saves CPU cycles.
Minor points. Saves some bandwidth (though of no issue to me). When going 1:1 it fills up the monitor at native resolution. With 3mp direct access going full screen I always had to scroll right to center, and down to get the lower more important part of the image, vs more sky/trees.
But primarily FOV, image quality and cpu cycles. Up to last week, I was dependent on software detection. Things start cooking really quick when you get a bunch of 3mp streams with software analytics on and live view. 2mp eased that to some extent, but in the end not enough for long term use, when you want to have one system do it all and will add a few more cameras.
That said, I may put a couple of them in 3mp and re-evaluate, now that I'm not dependent on software motion and have gotten the load sorted with ivms and substreams.