It really has a lot to do with how you plan to use the results. If you plan mostly on displaying your images online or not printing over 8 X 10 you will find 3Mp satisfactory – even after some cropping. If you want to print larger or find yourself cropping a good bit 4Mp would give you better shots.
There has been some question about the mechanism on the Z1 for displaying through the viewfinder for reliability. One of our posters said his friend’s broke the first day. One instance might not be significant, but it might not be as reliable if it gets bumped around.
Keep in mind that Oly supplies 4 NiMH batteries and a charger with the 750 but you will have to buy them for the Z1. The SD card on the Minolta is a slightly more economic choice than the xD on the Oly. Plan on buying at least 128Mb.