The way I see it, Saudi Arabia might oppress its own people slightly more than Iran does (Iran only deserves a little credit for its quasi-democratic institutions), but it's also much less of a threat to international peace. Yeah, Saudi Arabia does throw its weight around in the gulf (see e.g. Bahrain), and individual Saudi Arabians fund awful causes, but state policy doesn't support anything nearly as harmful as a WMD program or the "annihilation of Israel."
I guess if "being allied with Iran" meant that Iran stopped supporting terrorism and regimes like Assad's, I might consider that option. But Saudi Arabia would still have the advantage as a more important producer of oil, and as a more natural ally of other Sunni states such as Egypt.
All you're really saying here is that Saudi Arabia has more plausible deniability for the horrible things that it gets up to, and/or that the horrible things that it gets up to are insidious rather than self-evidently dangerous. Which we all knew already.
At least we can trust the people in charge of Iran not to bulldoze Persepolis to build a parking garage or something.