I don't really like either one, but here's my thinking:
If you think you're going to need a huge game from the WR position to win this week, I would take the risk on Miles Austin. I think he has higher potential for a bigger game. Atlanta's D is not very stout, to say the least, and while Austin isn't really a great WR, he's good enough to exploit a bad secondary that has issues with tackling (as we saw vs KC). The rapport between Austin and Tony has probably gotten better since that week, so Tony will probably throw to him more often. But that offense is as Jekyll and Hyde as they come. One week they look fantastic, the next they look like garbage. I think Austin is high risk but high reward.
I think Johnny Knox is the safer play. He's going to get a few catches, maybe a TD, he's been pretty solid for 5-8ish points depending on your scoring. He's playing the Bengals who just lost their number one pass rusher, which will make Cutler's day a little bit easier. He's also not the main guy the Bengals will be looking for. They'll probably have their eyes more toward Hester, and then Olsen in the RZ. That also puts Knox against Jonathan Joseph most of the day (I think so anyway) and he's not exactly a shut down corner (though Knox isn't exactly a number one WR).
I don't know what I feel Austin's stats will be, but I can see something like 70-80 yards for Knox, and maybe a TD.
Also, you seem to be new here, so just a piece of advice. Don't copy just the viewtopic part of the URL, copy the whole thing. The forum auto-formats to only the viewtopic as a link. If you don't do that, you make people copy/paste and I'm sure some people won't bother with that.