i feel they both end up being flat but:

the controls are different to enter them. I start a blender by bringing the plane down in a aileron only spin then i enter the snap position on the sticks (left ailerons, right rudder)

on a flat spin I enter with full L or R ailerons, and (L or R) rudder depending on it i do it to the left or right full elevator (depending on do I want inverted or canopy up. Some planes require crossing the ailerons to go flat 1/2 way thru the move, and vise vera with the elevator if you want to be inverted with cross controls on the ailerons, and rudder.

I would also say the blender will test the structure of your plane were as a normal flat spin doesn't put that same stress on the wings and air frame......

more 2 come from other flyers......