The poly urethane bushings won't be quieter than factory, but provide less squish and longer life.
What I would recommend is brand new factory replacement Ford parts to keep the noise and harshness to stock levels, but if you get the Energy Suspension ones, get the black, not the red. The black ES parts are graphite impregnated and will reduce the chance of squeaking.
Just out of curiosity, how old and how many miles are on the stock bushings, and how long have the new springs been installed?
2015 Mustang GT PP 401A Black | Roush Supercharger