I relied on a 334 for many years, until I obtained a 339.
The 333 and 334 have an extremely useful auto-balance function, which servos the fine frequency and fine phase controls to achieve null.
The 339 is great: a very low distortion generator built in, and the analyzer automatically tunes to that frequency. Most importantly, if the signal level changes within the selected range, the percent distortion function automatically re-normalizes and gives the distortion. With the 331 to 334, if you are adjusting for minimum distortion you need to re-check the input level (manually adjusting for 100%).