Well RPM has better eyes then the ion, and is capable of shooting faster. It has a better stock feedneck and trigger. But it has no va or reg. So ud need to get a VA and a REG. It is heavier then the ion tho the ion is plastic. It is louder then the ion. Also the stock barrel isnt great. The ions has alot more upgrades avalible. For more info on the rpm check out this thread.
http://www.pbnation.com/showthread.php?t=1338143