I have the Bilstein 5100's on my pickup and I haven't noticed much of a difference in the ride quality. The reason that I went with Bilstein over a spacer or extension kit is that I didn't want to be overextending the shocks. If you aren't going to be taking your pickup hunting, offroad, etc. then a spacer or extension kit will work just fine bc they are a little cheaper.
Here is a good explanation. http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0CCQQFjABahUKEwi6v7Ol_8HHAhXJKYgKHck8BYE&url=http%3A%2F%2Fwww.bilsteinus.com%2Ffileadmin%2Fuser_upload%2Fuser_upload_us%2Fpdfs%2FBil_LevShks_WebArticle.pdf&ei=2TLbVfruNMnToATJ-ZSICA&usg=AFQjCNGueTFPLWO0l0qAEsoguP8_ZXYzQQ&sig2=cPnfQV1SGWKigUr4Acl0IQ