Before I opted for MemberSonic Pro, I compared different membership site solutions. MemberSonic Pro is currently the only solution that seamlessly integrates out of the box with different affiliate systems, for me most importantly JVZoo and WarriorPlus.
Although this is promised by many other scripts as well, they often require an additional add-on for this feature.
More than that, MemberSonic is the only solution I could verify to automatically downgrade a member after refund or cancellation.
The low one-time fee for use on unlimited sites and regular updates are just the icing on the cake.