25+ inch monitor (Macbook Pro and Windows laptop)
This may have less to do with the monitor you get and more to do with how you hook up the laptops to the monitor. When using multiple desktops on a single monitor, I've found KVM switches extremely useful. If you were planning on using a single keyboard for the two laptops, I would recommend this approach. However, I'm not aware of a single keyboard that will give you OS-specific controls for both Mac (command, eject, volume, etc) and Windows (start, etc).