The largest difference is that the 3325A is much more precise than the 8165A - it's frequency, amplitude and phase noise specifications are much better, while the 8165A has a larger frequency range. As so much does, it comes to what you will be using it for and your specific needs. If you are only looking for a sine output, the HP 3335A really can't be beat for the money. Or, what has become my daily driver for low frequencies, the HP 3326A - more or less two 3325A's in one box with a lot of extra capabilities thrown in. Along with a HP 8663A, there is not much I can't do with the pair.
Hal