Best method ? hmmmh. Well that's always debatable , isn't it ?
Both the materials you mention have similar insulating properties, but the poly balls will, i think , cost a lot more. On the other hand they will fill up lots of little voids.
The presence of wires makes it more problematical. If the wires are PVC-coated, it is recommended that they do not come into contact with polystyrene ( risk of reaction, although this is fiercely debated) Equally it is recommended strongly that cables should not be covered by insulation because this reduces their power-carrying capacity and brings with it a risk of over-heating.
This is however a significant risk only with cables that carry a high load such as those for electric-showers and immersion-heaters.
If , as in just about every house, your cables are fixed tightly to the joists with no slack, you have the dilemma of how to do what you want ( insulate reasonably) without ignoring rules/recommendations .
Depending on how concerned you are, you could always insulate the garage ceiling by either sticking polystyrene boards to the ceiling or using battens/false-ceiling to support fibrewool, but's that's more work and more cost.
