Hi, if you read docs, weight just weights batch-wise. Theoretically you have no knowledge about what does you batch contains. I would go forward pos_weight in order to balance the contribution of each class.
pos_weight will be used when the specific class were chosen as GT (I  think, didn’t look at source code)
Sampling with assigning probabilities to each class is another different way of adressing this problem. However you don0t have to use both