I still think that Steeda jacking rails are the best invention for home mechanics. I can have the car up in the air and slip wood blocks or ramps underneath each wheel in seconds. No fumbling around on my hands and knees looking for jacking points, no worries about driving perfectly straight up onto ramps, etc. I have them on my Boss and my Focus RS.
I agree with this. They work well. I kinda hated bolting an extra 20lbs on the car that didn't make it go faster but ultimately it was worth it.
I've never been a fan of ramps. Even with a lowered car you can still jack on the diff first and put jack stands on the axle (not sure about this on S550's). Then its high enough to get a jack under the sides.
I guess if all you want to do is change the oil, maybe ramps are easier. But I never just change the oil. There's always something else to do or check so I just jack it up, set it on 4 jack stands and then I have enough room to roll around under the car.