ManualResetEvent and ManualResetEventSlim both are designed so that they remained signaled after calling. This is typically for a very different scenario than AutoResetEvent.
AutoResetEvent immediately returns to the unsignaled state after usage, which is typically used for a different set of scenarios. From AutoResetEvents documentation:
Typically, you use this class when threads need exclusive access to a resource.
ManualResetEvent (and Slim) are typically used, however, for a scenario where:
this communication concerns a task which one thread must complete before other threads can proceed.
Since AutoResetEvent is most commonly used in scenarios where there are multiple threads sharing a resource, wait times typically would not be extremely short. ManualResetEventSlim, however, is really only intended when you know, in advance, the wait time is very short. If your wait time is not going to be very short, then you should use ManualResetEvent instead. See the documentation on the difference between MRE and MRES for details.
When your wait times are longer (which would be the normal scenario with AutoResetEvent), the “slim” version is actually worse, as it reverts to using a wait handle.