Released in November of 2009, the Omega caliber 3304 is based on the ETA 7750 (unlike 3301, 3303, 3313 that were based on column wheel Piguet movements). This automatic chronograph movement is used in the Speedmaster Date (Ref: 322.214.171.124.06.001). It replaced the previous caliber 1164.
The newer cal 3304 has one specific technical difference to the cal 1164 that might have a slight bearing on your choice – the date of the cal 3304 is adjusted using a push button at the 10 o’clock position, while the cal 1164 uses the regular crown adjustment. I don’t believe that one is better than the other, it’s rather just a matter of preference on whether you want to use a pen or similar to change the date.