How to decide when is use ERD or UML for data modeling?
I think you answered your own question. E-R diagrams are meant to be problem solving diagrams for the sake of creating a database schema, whereas UML are supposed to model a completed db. I personally don't place a huge amount of value on ER diagrams, as I would personally do such brainstorming in my head, but I imagine many people would swear by them. As such, the answer is UML as a general rule, but ER can hypothetically be useful in the construction of databases.