The main difference is that the Rev-i is a spool valve marker and the FX is a poppit valve marker. Poopit valves are usually know for their efficiency since the ram opens up the airway so that the right amount of pressure flows in to the gun without wasting that much air. The spool valve on the other hand will allow the whole bolt chamber to fill with air and when you fire it will "dump" all the air in the chamber out to propel the paintball. To my understanding most spool valve markers will get around 6 pods while most poppits will get around 9 pods or so.
On the other hand spool valve markers are usually quieter than poppits.
I'm sure someone would go further in depth on both sides but most things come down to personal preference. Either way you choose you will be receiving an awesome marker although my vote goes to the Rev-i because it's too dang sexy