Neither.
I doubt there are enough people that would buy DB at this point to offset the cost of creating a decent AI. (And we all know that they will be ripped endlessly for the AI if they do create it

) How many sales do you think a single player AI is going to generate? 10? 50? 100? How many people bought the game over the weekend when it was $5? $5.00!
I have logged on 3 of the last 4 nights (-6) and have yet to see a single player online. I am in a couple leagues so I have scheduled matches but other than that there are *no* players online most evenings. Why would they put more time into a nearly dead game? (I'm just praying that we get private league functionality so those that have already purchased the game actually get to enjoy it for a while longer.)
(Just to be clear, I think it probably *should* have been released with an AI because it is a nitch game and "online only" can pretty easily turn into "none" if you don't have a really large player base. I just think that at this point in the life cycle they have collected a large majority of the $$$ that is out there for DB and adding single player AI will most likely not be cost effective. However, if they were to release it as DLC with a charge, it might work but I'm guessing the sales would still be quite small.)