The 1084 module is considered by many to be a 1073 on steroids. It can be described as a 1073 with some added flexibility, without losing any of its many famous sounds.
The differences with the 1073 module are as followed:
The Low Pass Filter is especially useful for sweetening overhead tracks, harsh or brittle hi hats and cymbals. It also helps remove unwanted hiss and noise on bass heavy tracks, where a flat high frequency response is not needed.
In combination with the High Pass Filter, Special Effects can be archived, like telephone, radio and TV sounds that never sound natural using plug ins!
The Lo Q option in the mid band makes heavy boosting exceptionally natural and helps tracks sound unprocessed. Acoustic and classical recordings will highly benefit from this feature.
Whether you're in doubt which module to choose, 1073 or 1084, and money is not the problem, the 1084 should be the way to go, as all the features and sounds of the 1073 are intact.
Whether you can live without the sweet Low Pass Filter and the 12Khz high shelving frequency limitation is enough for you, going the 1073 way might be enough.