I am thinking of buying an RFM69HCW FSK module from Adafruit. However I am confused as to which module to buy, the 433 MHz or the 900MHz version. Are both allowed in the UK? If they are then why do they make two different versions? Does one transmit further than the other or does one use more power than the other? There is also the option of a RFM69X, which again has the option of two bands, but this time operates on LoRa instead of FSK. I understand that this one can transmit further than the FSK. Any help is much appreciated.
Different bands may or may not be allowed in different countries, or not with the same rules (power, duty cycle, etc.). Also the exact band may vary (for instance the “900 MHz band” is actually the 868 MHz band in the EU but the 915 MHz band in the US). Some devices may support both, others only one (the RFM69HCW supports both).
Lower frequency bands usually have better propagation through obstacles, but regulatory limits (bandwidth, duty cycle, max power..) may be different. IIRC the 433 MHz band is quite limited (even more than the 868 MHz band, which is already crippled).
LoRa can have significantly higher range, at the cost of very very very slow transmission in some cases, with very limited duty cycles, which means in those conditions they are suitable for sending a few bytes here and there, not much more.
Note however that the largest factor for distance is unobstructed line of sight between the two antennas.
Not knowing your needs (distance, indoors or outdoors, line of sight, amount of data, intervals between transmissions, power consumption…) it is difficult to advise one device/tech/band over another.