Serrated blades come with sharp teeth lining the cutting surface, producing a saw-like motion when cutting through meat. Generally speaking, serrated blades are preferable when you're cutting through lean meat or thicker than average slices. Additionally, serrated blades are superior when it comes to cutting bread and some types of vegetables.
Smooth, or non-serrated blades are typically much thinner and produce a significantly finer cut than their serrated counterparts. For cutting thin slices of fatty meat, such as salami or prosciutto, a smooth blade is heavily preferred.
Neither is outright better than the other, and both have their applications. Because of this, most meat slicer enthusiasts end up purchasing one of either, allowing them to switch when the situation called for it.