A sliding oven is meant to fill the gap between two cabinets in the kitchen. The height matches the standard countertop while the width is slightly narrower than the top. Many feel that it is more aesthetically pleasing as it provides seamless visuals of the backsplash.
Freestanding ovens are different in that there is a protrusion at the back. This is meant to keep the dirt from falling backwards and protect the wall from the heat. The downside is that there is no visual continuity with the backsplash. This backguard often contains control mechanisms for the unit.
The choice between the two is a matter of aesthetics and budget. Sliding units are typically more expensive than freestanding ones. The difference is about 10% to 15%. Whether this is worth the benefits will depend on the priorities of the individual buyer.