Currently you can either specify a KeySelector, tuple positions, and expression keys? I think KeySelectors are strictly superior and with lambdas (or function references) quite easy to use. Tuple/expression keys use reflection underneath to do the field accesses, so performance is strictly worse. Also, when using a KeySelector you will have a meaningful key type KEY in your operations while for tuple/expression keys the key type is simply Tuple.
Tuple/expression keys were introduced before Java got support for lambdas in Java 8 and before we added the Table API. Nowadays, using a lambda is little more typing than using an expression key but is (possibly) faster and more type safe. The Table API should be used for these more expression-based/relational use cases.