[Demoquin]

Single weapon is when u are using a one handed weapon then ur ac will be lowered 1 to 2 points depending how many u put on the proficiency.
Sword and shield is when u are using a shield and a one handed weapon u get a higher ac against missles only like arrows and maybe magic arrow.
My suggestion is that if u are using a bow or x bow use the single weapon if not use shield with the sling.