Its really isnt a comparison. Spacers do not improve the ride, cause extra wear on joints, limit UPTRAVEL, overextend stock components. A .5" spacer wont be as bad as a 3". A spacer does nothing for performance except lift the truck. A spacer can cause stock shocks to be over compressed causing shock failure. This has been proven time and time again on many make and models of trucks.
A bilstein, regardless of setting, does not limit travel up or down. It will provide extra suspension articulation over stock, allow for full suspension range of motion, improve the ride with better valving and dampening characteristics, allow for better dampening on rough roads as they are monotube shocks vs a cheap twin tube. As they use the stock upper mount, the shock angle is in a better position which will keep the coil from hitting the arm.
I posted a link that answer all of these questions above. PLEASE READ THIS - http://www.bilsteinus.com/fileadmin/user_upload/user_upload_us/pdfs/Bil_LevShks_WebArticle.pdf