Newbie here, but have been reading the boards for awhile.
The niners are 27th in pass defense and the Cards at 29.
On the Niners defense the most important factor is the absence of CB Nate Clements and the lack of a dominant pass rush. Although Nate is having a bad year he is always money against Cardinal receivers and plays his best when matched up with Fitzgerald. Some may say Shwantae Spencer will fill in nicely at CB because he came from the same school as Larry and went up against him at practice every time, but I don't think it's the same. In addition, the safety positions are bad.
Frank Gore is no longer a running force, although comfortable catching, with the terrible offensive line of the niners. This is one of the reasons they switched to a spread along with the strengths of Smith. The Cards say they will target Gore but I don't buy it, they will be gunning for Alex Smith who threw for over 300+ yards last week against the seahawks who have a slightly better pass defense than the cards. The last time these two played the niners won 20-16 but they were a different team, a smash mouth clock eating running team. Look for the niners to utilize Vernon Davis and Delanie Walker, arguably the two fastest tight ends in the league, along with the budding chemistry of Smith and Crabtree.
I see this as a shootout with both teams being stout against the run.