For the Billies at least, lift isn't achieved due to a longer strut it's due to the springs. Angles aren't entirely the same in the case of Bilsteins either, 1.8" =/= 2.50" there is a reason Bilstein went with 1.8" you could argue that angles are nearly the same with a 2" spacer which is a better comparison, but your still extending the strut with the spacer. Having ridden in both and been in both i'll take my Billy's every day of the week over spacer kits. Of course there are struts that are much longer, but Bilsteins are 1/4th an inch longer than factory. My brothers truck rides like crap (RC2), My cousin's the same (RC 2.5). At the end of the day it's preference and what your willing to spend. Had i had it to do over again i just would've went with more and waited on 6112s and gotten rear shocks with reservoirs. In the end they both work, i vastly prefer one over the other though.
http://www.bilsteinus.com/fileadmin/user_upload/user_upload_us/pdfs/Bil_LevShks_WebArticle.pdf