Warner would make the niners a lot better, but he won't be nearly as effective there as he was here. Don't forget, we're not that far removed from the turnover-prone Warner. I think the system and his WRs had a lot to do with his success here.
Now, that said, I think he's a better QB right now then Leinart and the Cards will be worse of next season should he leave. But, maybe the Cards will be better off in the long run because Matt starts getting some reps under his belt with what looks to be a consistent philosophy and coaching staff.
So, in short: if Warner stays, great. We could use him next season. If he goes, that's okay, too.