STI vs Polymorphic Associations

I have a MasterEvent model. It has attributes that is required by
another model Event. The user selects the MasterEvent model and I
create the Event model by merging the attributes of the selected
MasterEvent with the attributes that were presented for the Event
model in the form.

I have association declarations in the MasterEvent and Event models.
If I use STI does the subclasses inherit the association declarationed
in a superclass? Should I use polymorphic associations? I read the AWD
book but still not clear on these issues. TIA.