Q flat or round shader

I am new to this group so please forgive me if this question has been
asked before but i was wondering if anyone could tell me tell me the
difference of shading/coloring  with a flat needle configuration as
opposed to a round shader. Is this just an artist preference or is there
any major difference. I have had many tattoos done with both needle
types and i noticed that the flat seems to hold color better on myself..
I am getting sick of going back for touch ups so i was wondering if it's
the artist who sucks or the needle configuration. Any advice would be
very much appreciated.
MIKE