Kush and Axum were similar in that both were commercial and trade centers. Both civilizations were strategically located for trade: Kush was positioned along the Nile River, facilitating trade between sub-Saharan Africa and the Mediterranean, while Axum was located at a crucial point along trade routes connecting the Arabian Peninsula with Africa, allowing it to control trade in goods like ivory, gold, and frankincense.
While the other points have elements of truth, they do not apply equally or as directly to both civilizations. For instance, while Axum was significantly influenced by Islam after its emergence, Kush existed prior to the rise of Islam and was influenced by different factors. Additionally, both civilizations did not rule through a caste system; their social structures were different in nature. Lastly, while the trans-Saharan trade route was crucial for many African civilizations, it was primarily a feature of later historical periods and not the main focus for both Kush and Axum simultaneously.
In summary, the best answer is that both were commercial and trade centers.