higher or lower gauge for 3-6 feet XLR cables?

For signal level voltages, either is OK, The 20 AWG is more typical.
16 AWG is a bit of overkill IMO. (for interconnect level voltages)

For folks who are certain more is better... then 16 must be the one. Otherwise 20

The higher the gauge the smaller the wire; smaller wires have higher resistance. There is very little agreement on wire size; some [Mapleshade for example]use very small diameter, others use larger ones. The size alone will give little indication of the sound quality. Some like silver, I use it sometimes but usually use carbon or copper; it is a matter of taste and system matching. I would get ones from a company with a good reputation and not worry much about the cable size.

20 guage is more than enough. Some would say 28-30g but could be fragile to handle. Solid silver does seem to be the way to go however. I'm very pleased with my homemade XLRs.