I'm not a huge fan of the all in one design of the iMacs, hence I use a Mac Pro. For editing large format scans the most important factor will be the amount of RAM you have. So get a system that can be expanded past 8GB. I have 24 now and it feels like enough, even with several large scans open. With 8GB I was frequently shutting things down, or closing images in an attempt to manage memory. I have better things to do with my time than managing memory.
Mac or Windows really doesn't matter once you are in Photoshop. But frequently Windows machines are made with lesser components to keep the price down, so be sure you really know what you are buying. With like hardware Apple is still a bit more expensive, but nothing like it sometimes seems when you compare a bargain windows machine to Mac.