Taking Amtrak to Burbank is pretty straight forward affair. Your hotel probably has a shuttle that can take you to the Anaheim train station (you should check with them).
If you want to drive, ONT is the way to go given your options. Traffic on I-10 or CA-60 going east towards ONT will be rough on Labor Day. Some holidays result in light traffic on those freeways, Labor Day is not one of those holiday.