There are pros and cons to both serrated and plain edged steak knives. Steak enthusiasts claim that serrated knives tear at the flesh of the meat which causes all the flavorful juices of the meat to flow out of the meat and on to the plate. A plain edged steak knife prevents this from happening.
Serrated knives are also hard to impossible to resharpen to their original factory condition. Very few store bought sharpeners have the ability to sharpen serrated knives. Even your local professional sharpeners most likely will not have the specialized equipment to sharpen serrated knives. The micro serrations that you find on a lot of steak knives cannot be sharpened at all. Plain edged knives can be resharpened much more easily.
Keep in mind though that you are most likely going to be cutting your steak on a ceramic plate. Ceramic will dull a plain edged knife in no time. A serrated knife will stay sharper much long than a plain edged knife even cutting on a ceramic plate.
Therefore, if you are a person who is comfortable frequently sharpening your steak knives, opt for the plain edge. If you are like most people and avoid sharpening your knives at all costs, opt for the serrated.