ReaderT (IORef MyState) IO a is introducing a global mutable state into your IO actions. This style is discouraged, because you have less guarantees about what can happen with your state. For example IORef is not protected from multi-threaded access. If you try to use this state in multiple threads you may run into bugs. StateT MyState IO a doesn't have that problem. For a typical use-case, that is an action that: reads state, performs action, modifies state, both are similar and neither should be significantly more comfortable. I would recommend to use StateT version unless you find that either it does not fit your model of computation or that it is underperforming. As for the performance it is hard to say. StateT allows the compiler to reason more about your code, so it might optimize better. However that's only one factor. You would need to test it to find out which one is better.