For your research question, "What are the social benefits and liabilities for tribes developing casinos on Indian reservations?" the more authoritative source would be:
a peer-reviewed article published in UNLV Gaming Research & Review.
Peer-reviewed articles are typically more credible as they have undergone a thorough evaluation process by experts in the field. In contrast, an article published in Sports Illustrated is likely to be more focused on entertainment and sports rather than providing in-depth academic analysis relevant to your research topic.