The new Backs (beyond 16MP) are all same size. Phase One & Imacon/Hasselblad uses Kodak sensor, while Leaf even though owned by Kodak uses Dalsa (Phillips, I think) sensor. Leaf is more softer, filmlike look, while Phase One, with it's Capture One software more saturated look. Michael & evryone here prefers Phase One, though there may be a few shy Leaf users. Imacon/Hasselblad users do not get to voice their opinions too much, do a search in Rob Galbraith Forum and you will find that to be true. They usually hang out down under in an extremly popular Yahoo user group for Imacon, which is run by Nick Tressider at http://www.nick-t.com/
Actually it is very good & quick in responding other users' problems & querries.
My opinion is all three (Phase one, Leaf & Imacon) are same. If you are a Canon user, who uses Capture Software than you might prefer Phase One. So honestly use of software and how comfortable you are with the sales rep, probably will dictate your dicision. Only difference I see is LCD on the back. Aptus being in class by themselves. Hasselblad's new back has also excellent bright LCD, though smaller than Aptus, but better than Phase One. Phase One's LCD for outdoor in sunlight really sucks. But if you are shooting in studio than it doesn't matter. Obviously Sinar looks real good, but only thing I know@ is it internal 6GB capacity, which has its phenomenal merits. But the best is try out all 4 backs and decide which is for you. Good Luck.