Some more info for the 5100 vs 4600 debate. I got this reply back from a Bilstein rep when I was trying to make that decision myself. As mentioned above, I went with the 4600's and am very happy with them.
"I'm leaning toward getting the 4600 shocks unless there's a reason I should go with the 5100 series. I frequently tow a 6500 lb trailer.
If your vehicle is stock ride height we would highly recommend our B6 4600 series. They are considered by most to be an upgrade in comfort and performance over the OEM system and provide superior handling and control when towing. At stock ride height they are our best riding and longest lasting damper. There is a lot of misinformation online that suggests our 5100 series is stronger, better or more heavy duty than our 4600 series. Nothing could be further from the truth. Both are made out of the same high-quality materials; the biggest difference is the coating and the fact that the 5100 is physically longer to accommodate for a lift."