If you want to get a certain weapon quicker without grinding yen, buy by robux. If you want to farm yen faster, go for the game pass. However, I would go for the yen game pass, so you can keep it and keep grinding yen faster every time.
If you plan on changing frequently and earning other items faster, then 2x Yen may be for you. If you only want one item, then buy the item with robux.