In UAT vs QA, which one is the most effective?

UAT vs QA

Both have different goals.

From a commercial stand point QA is there to make sure the clients will accept the version during UAT. After UAT, taking it into production and making sure the client does not run into (too many) critical issues. Client UAT is there to verify clients do not take a version into production that does not meet their minimal required workflow and make sure that they can continue working even when running into minor issues, not spotted at first.

From a personal standpoint the QA assists on the development teams side and the UAT is only on the client side. Both want the be able to assure the best product possible. The client just should not trust the dev team's QA. The QA team often focuses on general needs of multiple clients and issues that have arisen in the past. Where clients could be using the product in ways that are not specified and need to make sure these "workaround" features still work in newer versions. QA helps the dev team in having a fast feedback (quality) cycle for new features and to safeguard older features with automated or manual test cases.

UAT makes sure the client teams can work with newer versions. Also, there are different phases in the DTAP model, where QA is the T(est) phase and UAT is the A(acceptance) phase.