Question

what is better? is there any difference in runtime between the two options?

Answer 1

There is no runtime difference between locking a static and locking an instance member. However, it can break your code if you use an instance lock and are updating a static.

class Broken {
  static int myCounter;
  object synch = new object();

  void SomeMethod()
  {
      lock (synch) { // BAD
        ++myCounter; 
      }
  }
}