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I think most people get H&R spacers, but I'm not sure. Vogtland and H&R springs drop the same amount, and I believe look exactly the same (just one is purple and the other is blue)... don't think there's really any difference to speak of, just go with the color you like. H&R might drop a little less in the front than Vogtlands. I have Vogtlands, no complaints. 1.6" drop all around, maybe 1.7" in the front since I have the heavier E550.
Oh and I forgot to mention, with the springs, I didn't need an alignment. I get it checked every time I take it in, and they always say it's up to spec... have two sets of wheels, so haven't gone through a set of tires yet to know if I have uneven wear. One thing I noticed with the springs is that the shocks are a little more bouncy now since I guess the spring rate is off... a stiffer shock would be ideal and make it less like a '60's Cadillac (it's not THAT bad, but I notice it)
I would suggest first getting a set of 2 15mm H&R spacer with OEM 18" wheels and put one on the front and one on the back and drive it around the block because I really believe the 20mm will rub against the fenders which you don't want unless you are willing to roll your fenders.
I have H&R spring will bottom out sometimes going over speed bumps with 20" wheels, but if your going with spacers, it basically means the same thing as 20" wheels so I would go with the other.