Got to be the Cards, for a few reasons...
1. My number one rule for divisional games - if it's unclear who is better, root for the home team.
2. It's unclear who is better because the Niners are 1-1 against two potential playoff teams; Cards are 2-0 against two of the worst teams in the NFC. Based on resume, the Niners is actually a bit more impressive.
3. It's more impressive because the Steeler game meant nothing - Niners played a late Monday night game, then an early Sunday game. I can't think of a single team that would hang with the Steelers in Pittsburgh in that scenario. Which means that it's conceivable that the Niners could still win the NFC West this year.
So, even though it appears right now that the Cards are better, it would be worse for the division overall if the Niners won this game....because you have to assume they'll be the Cards in SF later this year....so winning this game would almost be like winning two divisional games. But the most important reason of all, logic be damned, is that the Niners are the Rams biggest rival....and they should lose every game they play...terribly.