what is difference between built in ads (Services ads) and Unity Ads SDK 2.0? which one is better?

i am having error while using services ads!! ERROR : Assets/Scripts/ads.cs(4,19): error CS0234: The type or namespace name `Advertisements' does not exist in the namespace `UnityEngine'. Are you missing an assembly reference? I am using Unity 5.5.2f1 and am building for android!!