The Simonton is a well regarded window by most pros.

Any additional details (model number, series, glass options, etc.) you can provide on the United would probably yield a bit more feedback.

United window is an average window( at best ) which is basically made for contractors, same with simonton. not a fan of either, especially United. depending on where you are located, i would search for higher quality/more energy efficient brands such as Okna or gorell.
both are available through dealers that also take care of the installation which usually results in a high quality install.