I chose the RB67 because no batteries. If 6x4.5 is desireable, i switch the back. An M645 is only 6x4.5 as far as i know. Although a good point has been made about shooting hand held. The RB67 is heavy. I have the hand grip, prizm view finder, and have used mine hand held. Wouldn't want it around my neck all day, that's for sure.