Twin needles can be used in either size according to your preference, they both perform the same function it is just whatever you prefer visually.

I like narrow (2.5mm) hems for leggings and wide (4.0) hems for waistbands. Same thing for swimmers, I prefer the narrow for leg finishing and wide for waistbands. The narrower needle will give less tunneling between stitches on lighter weight fabrics.