In addition to better temps, the Strix is also slightly faster, and Asus has better QC and great customer service records compared to Zotac. Thats where the additional cost comes from. Bear in mind you can transplant that system into a better case though you may also require a new PSU. Dell is notorious for having low quality proprietary power supplies.
Also note, fanless really does not mean it does not need airflow, it just means it will use the case fans airflow to cool the card, and yours does not have much.