Will there be a difference in RPM ?
The main difference is PWM allows operating fans at a much lower RPM than voltage control does. That's because the "bursts" are always full 12 V which can kick the fan to start rotating. The duration of the "kick" just gets shorter (percent of time) in duration to lower the RPM.
DC control just lowers the voltage which doesn't kick it into starting when it gets below about 5V or so. That can damage the fan if left not rotating with 4 or 5 V on it for extended periods so many fan controllers won't give it less than 6 or 7 Volts minimum.
It's less a big deal these days with the ultra-quiet fans made by BeQuiet, Noctua, Arctic and others as the operating RPM at even 9 or 10 volts is whisper quiet.