"Soft wood" variety. They are sharper and have a shallower bevel.
EDIT: The v-nails in polystyrene are really only acting as internal clamps until the solvent/glue dries. I have never seen a properly glued polystyrene frame break at the joint regardless of the kind of mechanical fastener.
I agree with Wally. And here's one more bit of information:
Poly mouldings are dense, something like MDF (no reference to hardness here). When the v-nails go into the material, it does not compress as wood does. So, each v-nail acts like a wedge, and too many v-nails could cause a separation at the outside of the corner. One way around this is to glue & vice the corner, then let it dry before v-nailing. If that's not practical, then be cautious about placement, and using too many v-nails.