The Advanced system is half the size and weight and also half the battery capacity, so it can be monitored for 45 days instead of 90-days. Or be in idle mode for 15 days instead of up to 30 days with the Classic. The Advanced tag is much more waterproof and is Ok for prolonged swimming whereas the classic is good for rain and snow and an occasional brief swim. The Advanced tag is also designed to be more durable for dogs that run through brush or play rough with other dogs.
If you have a small pet or a pet that swims the choice is obvious, you want the Advanced tag. If you have a medium sized dog that doesn’t swim or roughhouse with other dogs then the Classic tag is a fine choice.