The thickness of the shaft usually varies between 6mm and 8mm. The thicker the shaft, the more power it will carry. So, if you aim to catch some small or medium-sized fish, go with a 6, 6.5 or 7mm shaft, whereas 7.5 and 8mm options are better for larger catches. Remember though, that if you hit a rock or a reef with the thicker shaft, it is more likely to get damaged. Thinner shafts are better for beginners and will most probably survive various collisions.
Spearshaft tips can be either fixed or threaded. If the tip is fixed, it is an integral part of the shaft and cannot be changed. Therefore, in case you damage the spear tip, you have to replace the whole spear. Some advantages of fixed tips are that the spear is usually more streamlined and damages the fish less. If the shaft is threaded, it basically has no tip and gives you the advantage to use any tip you want and change it as needed.
As for the tip style, there are essentially two main options - tridents and floppers. Tridents can have 3,4,5, or even 6 points, depending on the type (standard vs. flat). They are great for immobilizing the fish and more forgiving to bad aim. However, tridents are less hydrodynamic and travel well only on short distances. Flopper tips are more streamlined and can travel far. They are designed to anchor in the fish and keep it from escaping. Such tips can have one (Hawaiian or Tahitian), two (symmetric or nonsymmetric) or three floppers.