I would use the TL431 shunt regulator with an optocoupler for the feedback loop. The regulation is much better. Most SMPS control IC vendors have app notes covering this method. I am currently using such a method and with great results. The auxiliary winding off the transformer is not a bad way to sense voltage if the line and load regulation needed is not too tight. With winding feedback, 1 or 2 % line and 2 to 3 % load regulation can be expected. With the TL431/optocoupler approach, I get line and load regulation of around 0.2%. Just my thoughts. I don't know how precise your output needs to be, but the TL431 is the tighter of the two regarding regulation.