There is virtually nothing that has ever existed in science-fiction film or television that wasn't around in prose SF at least a decade earlier, usually two or three decades earlier.
Definitely! Neither show was "original" when their objective elements--things like methods of faster than light travel, shapeshifters, space stations, etc.--are taken into consideration. If anything, the similarities are interesting only from the perspective that Deep Space Nine stole Babylon 5's thunder: that B5 had a claim on the sci-fi tv audience of the mid-90s, but Trek edged it out. I'm sure Straczynski was also annoyed that he had to share the resources of the entertainment industry with DS9: the number of people in TV who worked on sci-fi was quite small (although much of that reflected the success of TNG).
Moreover, I don't think that the two shows competed against each other as much as is suggested. I moved around a lot in the 90s, and everywhere B5 followed DS9. Back to back, I was willing to sit down for an extra hour. If anything problematized B5's success, it was the explosion of syndicated TV and the formations of both CW (which along with B5, was Warner) and UPN, which ate up good time slots. DS9 was not immune to the same problem, though.
Regardless, genre shows tend to converge toward one another. On the one hand, they work through the same ideas and mimic one another in their competition with one another (keeping up with the Joneses, y'know).