I had a look and they seem to be about the same profoemance wise and the warranty is the same for all. For what it is worth you could buy two CF2400's for the price of an FX5
FX5 - 2300L/h, 5.9L Capacity, 3 year warranty and sutiable for up 1500L aquarium
Pro3 2075 - 1250L/h, 9.2L Capacity, 3 year warranty and sutiable for up 600L aquarium
CF2400 - 2400L/h, 22L Capacity, 3 year warranty and sutiable for up 800L aquarium