If the condition is uniform (i.e. constant for the entire pass), then the branch is essentially free because the framework will essentially compile two versions of the shader (branch taken and not) and choose one of these for the entire pass based on your input variable. In this case, definitely go for the if statement as it will make your shader faster.
If the condition varies per vertex/pixel, then it can indeed degrade performance and older shader models don’t even support dynamic branching.