We've gone for Labor Day weekend the past 2 years. Saturday and Sunday will be relatively busy. Monday is not too busy at all, and you probably would not need Express Passes at all on Monday.
The main differences between Express Pass and Fast Pass is that Express Passes are not free. They are available for purchase. The good news is: 1) you can wait till you get inside the park, check the wait times for the rides you want to do, and then purchase them if you think you need them. 2) Unlike Disney's FP, you don't not need to ride within a time window. You purchase the express pass and simply use it any time you feel like it.
Of course, if you stay at one of the 3 on-site hotels, your hotel key acts as unlimited express.