使用视图模型或编码ui测试进行单元测试

Currently i want to improve out test cases. Since we had switched to WPF with MVVM i'm thinking about writing unit tests to work with view models (test view models) or better use coded ui tests. What is the choice to take, or is testing both things the way to go? Currently i could not find any practical answer, maybe some one has a straight forward answer.Currently i want to improve out test cases. Sin