There are so many factors that make up a monitor's image that it is hard to do a comparison anywhere other than a controlled setting. At the minimum you must test both monitors on the same computer, and be sure that each monitor is properly adjusted. Testing just one of each brand will not necessarily yield results that can be generalized to all models and sizes of a given brand. The best way to select a monitor is to learn how to properly adjust a monitor and then once you have personally adjusted the monitors in question, select the one with the most appealing image to you. The purpose of degaussing is to demagnetize the shadow mask or aperture grille to allow the electron beams to pass freely through.
Most monitors automatically degauss when switched on, and many have manual degaussing buttons.
To find out if your monitor has this feature check the user's manual or manufacturer's data sheet. When you manually degauss a monitor the image will wobble for a moment indicating that the degaussing circuit is working. If the purity does not clear up wait at least 10 minutes before degaussing again to allow the degauss circuitry to recharge. If the purity problem persists a qualified technician can degauss the monitor with a special tool. The standard DB15 connector used on most monitors is adequate for nearly all applications.