Head pin with a rounded hat this gives it a beautiful ending and does not give the edge of a regular head pin. The 0,51 mm head pins are the thinner one and easiest to work with. For small beads with small hole this is the one recommended.
Choosing the length and thickness of hat pins depends a bit on what they are to be used for and which beads you choose. Many times, it is also taste that decides the choice.
0.64 is the slightly coarser hat stick and is experienced by some as hard to work with but instead gives a more stable result compared to 0.51 To keep in mind is that many really small freshwater pearls do not go at 0.64 but require a thinner hat stick, then choose a thinner stick.