The Date and Datejust are two separate Rolex lines, probably marketed to two different demographics. From a purely physical standpoint, the Date case is 2mm smaller...the bracelet 1mm smaller at the lugs (19mm instead of 20mm). My terminology would be that the Datejust is the flagship model of the Rolex line from a historical standpoint. Both have shared the same movement through the years. (BTW, the introduction of the Datejust model was not the introduction of the quickset movement...the quickset movement is a relatively 'new' function of the 30XX series movements, as the 15XX series did not have the quickset feature...it did, however, have the 'hack' feature that allowed the stopping of the sweep second hand for time setting. Prior to the hack feature, one would have to 'retard' the sweep hand in order to get the second hand to align for exact time setting).
It is the Datejust that has the larger number of dial options and the metal options as well. Solid gold, two toned and stainless are all offered for the current Datejust, while the Date in its current incarnation (newer style case and bracelet) is only offered in stainless (but optioned with a white gold bezel). The prior series of Date models had the solid YG, Two Toned and stainless options.
The Date has a much more limited dial option set.
One is not 'better' than the other...probably out in the non Rolex fanatic world, there's not a whole lot of differentiation between the two...buyers probably are steered towards the DJ because of the much larger number of colors and styles. Sharing the same movements, they are both certified chronometers.
Although I own a Date model from the late 80's, and enjoy wearing it very much, I fully realize that here on internet watch forums, the 34mm size is now considered a small size, while the DJ is considered a more accepted size at 36mm, or 39mm in the DJ II models.