if (string) vs if (bool) in C#

Is there a noticeable difference in performance/code quality if you use a string to compare at the start of an "if" instead of a boolean?

Example with string:

string isTrue = "true";
if (isTrue == "true"){
  // do something
}

Example with bool:

bool isTrue = true;
if (isTrue){
   //do something
}

I generally use a bool for these kind of comparisons but I've seen both variations online.