I've never posted on the forum before, but noticed this thread a chuckled a little. Although the comparison is a little odd, it's one that I've made for myself in the last 6 months. I can offer a little bit of perspective having owned a 2012 RR SC and now a 2013 550ix. As prelude to what I'm about to say, I've owned the RR SC since new and held onto it for only 2 months before throwing in the towel. Both cars are dramatically different in the way they feel and the way they drive.
The RR SC was beautiful to see, looked great in the front row of the valet, and drove as comfortably as any SUV I've ever own. It turned well, stopped well, accelerated well. Compared to the 2013 Cayenne S, it was more comfortable, more luxurious, faster in the straights, but lacked steering feel compared to the Cayenne. The RR looks like a 70k+ car. The Cayenne S doesn't seem to look as 'special.' I also felt the RR was much more car than the X5, with the exception of the X5M. The RR was a great car except that it SQUEAKED incessantly. The tech was patient and resolving some of the squeaks and rattles, but could not resolve all of the them. I finally gave up. The car was brand new and sounded like it had 60k miles on it. A friend has a 2010 RR SC and the squeaks were worse. This was not a good sign since the problems will only get worse in time.
The 550 is a much different car. It's much quicker than the RR SC. It handles better. It's also can't offer the 'cool' factor. Baller's don't drive 550's. It's more adaptable to upgrades. It has fit and finish many levels above the RR. It offers a more satisfying commute for me. There is much talk about the current gen 5 series not offering the BMW feel, the connection to the road. I would tend to agree. But those issues are easy to resolve if you are willing to do some mods. Even in stock form, my 550 with the M sport and DDC feels better than any comparable sedan in the class.
The choice in cars is very personal. If you decide to go for the RR, my advice is that you scrutinize the car that you are considering. Make sure that every panel, dial, switch, etc is perfect. Make sure the mechanicals are noise free, and the car drives like a vault. As others have said, get rid of it before the warranty ends. The tell-tale is that most extended warranty companies won't offer a policy for RR. I'm not advising you get an extended warranty. Rather, it might be wise to acknowledge what that implies.
Good luck on your decision. I'm sure you will love whatever vehicle you choose.
2013 550ix M Sport and other goodies