I wouldn't (and haven't) used any of those 3 methods. I'd (and have) do the clustered index by Date (the partitioning column) and the PK. It's going to happen anyway because the PK is unique. And make sure it's a unique clustered index.

If the data in old months is 100% static (think "audit table" or "closed invoice detail table"), then also consider making the DATE partitions live on 1 filegroup/file per month and make them Read-Only so that you don't have to worry about backing them up or doing index maintenance any more. It's been a real timesaver for me and will make "Piece-Meal" restores a whole lot easier if {knocking on wood} such a thing is ever needed.