BF3's SP campaign will be short (from what the developers have said) and (from what we've seen from the trailers) a scripted, linear affair. The makers of Brink, OTOH, have tried to ensure that bot play will be dynamic and different each time you play.
So Brink will have
ipso facto a longer-lasting appeal than BF3.
(Wait, people actually prefer to play online! As in, without bots?! :blink:
Well, I'm just not one of them - I've been playing SP for years and, besides, all you need to do is look at my Black Ops, BC2, or Crysis 2 k/d ratio to acknoledge my MP online svckitude...)
Still, Brink has enough uniqueness to at least form a lasting niche.