I find that finer needles cause less discomfort, and 31 gauge seems the thinnest available. Coarser ones allow smoother drawing of fluid with fewer air pockets forming. The smaller capacity syringes make expelling bubbles trickier by flicking. Yet they allow better precision when measuring tiny volumes. Ultimately it comes down to what feels right for me. Most often I go with the 0.3cc size in 31 gauge, though the 0.5cc version handles all the same tasks. When dealing with larger amounts like 50 to 100 units of that supplement, I switch to a 1cc syringe with 29 gauge because pulling that volume through the finer needle takes longer and feels more difficult. I stick with one particular brand but have no idea whether alternatives perform better or worse.