Propagation comparison between 868 MHz, 915 MHz and 2.4 GHz
In general, lower frequencies have lower free space path loss and therefor with the same power, same receiver sensitivity and same antenna gain will have longer range.  However, as the frequency increases, antennas become more compact so if you are constructing a link with directional antennas higher frequencies can be advantageous.  

https://en.wikipedia.org/wiki/Free-space_path_loss

http://www.ti.com/lit/an/swra046a/swra046a.pdf