What is the difference between the AEM 30-0333 and 30-0334 wideband gauges?

The 30-0333 is obsolete; only the 30-0334 is available. The 30-0334 measures AFR with its own wideband sensor and adds that value to the vehicle's OBDII network, so other OBDII scanners or loggers can read it as a separate value. Make sure this is the function you need before ordering.