This is not the outcome-determinative test used in Guaranty Trust Co. v. York, 326 U.S. 99 (1945). There the court appeared to ask whether, in the middle of the case, shifting between the federal common law rule and the state rule would make a difference to how the case turned out. The question is instead whether the difference would lead to ex ante forum shopping.

The question is whether the difference would lead to forum shopping in general, not in the particular case. Would a plaintiffs in diversity/alienage cases (or cases with supplemental jurisdiction actions) in general choose, or avoid, federal court due to the difference? Would defendants such cases in general choose to remove, or decline to remove, due to the difference?

Notice that now the concern is no longer about state interests. The state that created the rule does not care whether it is used in federal court. It has not bound up its rule with its cause of action. But the twin aims could still recommend that the federal court use the rule that would be used by a forum state court. Notice as well that the issue is now uniformity with a forum state court, not with the state that created the cause of action. So if a federal court in New York is entertaining a Pennsylvania cause of action, the twin aims recommend uniformity with a New York state court, not respect for Pennsylvania interests.

The inequitable administration of the laws test is closely related. The idea is that it would be inequitable to have different procedural rules apply to two cases simply because one happened to be a diversity case (and so could be brought in federal court) and another wasn’t. In general if there is the likelihood of forum shopping a court will also find that there is an inequitable administration of the laws problem. I discuss the inequitable administation of the laws test more fully in my twin aims article (pp. 1896-1900).