Dante Stella's site on these discussed the wide compared to the normal. I have the 645 and the 645zi. Love both though could live with the 645 alone. But if 28mm wide perspective (in 35mm film terms) was my forte (its not I mostly prefer normal to short-tele) I'd buy the wide in a heart beat. Do you like and mostly prefer wide angle?
I find all kinds of shots that dont fit in the GA645 frame. It can be frustrating at times.