I am starting to think that Samantha Nicole Design is too long, and for my own records have been calling is S.N.D for short. Do you think that is a better shop name than Samantha Nicole Design? I think it may be easier to remember, but I wanted other shop owner's oppinions.
I would stick with Samantha Nicole Designs. You won't need a DBA since this is your real name (I'm assuming), you've already began building a brand with it, it is more personal than S.N.D., and actually easier for me to remember.
I would shorten it to Samantha Nicole. Too many shops add Design to the end and it sounds 'cliche' and doesn't add any information about what you do. Just my 2 cents.