The main differences: 08 and 09 have a spring reg instead of the old shim stack regs (more consistent, easier to clean); 09 has gotten rid of the 08's QEV block by integrating its function into the all new solenoid design; the 09 has a slightly raised rammer housing, reducing kick, and also a larger volume chamber thingy (my brain isn't working); also the 09 has a self purging reg that removes the need to degass your marker as it does this for you when you take your bottle off. If I were you I'd go for an 08, value-for-money wise.