I have used both. Menz 203 is an awesome one step for medium/softer paints. Medium cut and finishes down beautifully. Fixer has more cut and also finishes extremely good. Fixer works a bit faster at lower speeds. If the finish isn't quite to your satisfaction, drop your pad aggressiveness and use it to finish. Very convenient compared to changing pads AND polishes.
Between them, Fixer is more versatile but, honestly, you can't go wrong with either. If I could only have one though, it would be Fixer.