I find I can create bullets with the - and + both. How can I decide when to use + and -
They make no difference, they are just a style of bullet and have no meaning.
There are many different bullet types you can use, for sorted and non-sorted lists.
You can use
org-shiftright (s-right) and
org-shiftleft (s-left) to cycle through bullet types to see them all. It is up to you to decide which one you would like to use.
See also the docs of
org-cycle-list-bullet to see all possible bullet types.