One is 24" and the others 27" (the 24" also comes with an Nvidia 3D Vision kit if I remember Benq's naming scheme right), other than that they are basically the same.
Which one is better for you is going to come down to how far you sit from the monitor, which will change the perceived PPI (Pixels per Inch). If you sit closer, get the 24", if you sit further away, get the 27".
With the 27", you wont actually have any more desktop space as their both the same resolution. Same number of pixels in both, just they are larger (hence a lower PPI) in the the 27".