Hi,
The choice between heaters is determined by what your printer or electronics board on your printer can supply. Some printers have 12V systems and others have 24V systems. Your heater selection should match that. if you do not know what voltage your system is, please contact the place from where you purchased your printer.
The 300C and the 500C are determined by which temperature sensor or thermistor you purchase. Your Pico is capable of both temperature values, however, it is limited and controlled by which temperature sensor you choose. Not by which heater you choose. If you purchase the 300C sensors, then Marlin table #1 in your printer's firmware should be selected (if you have Marlin). If you have chosen the 500C temperature sensor, then you must download the temperature table from our website.
To further help you, can you let us know what printer you have and what software and firmware you are using?
Thanks
Mike
The B3 Team