Do the Borg believe that Data is inferior to them somehow?
Data's technology is demonstrated to be somewhat compatible with (and perhaps superior than) the Borg in The Best of Both Worlds, and in Descent Part 1, Data shows that he is physically stronger than a Borg drone.
Certainly the Borg want to assimilate all of humanity, however they have shown favoritism (the Drone episode from voyager comes to mind as an example) when there is a particularly "juicy" target. Does Data not qualify as such? or is the entire enterprise at the same level of "juiciness" for the Borg?