I do Tosfos. Most Rishonim, Achronim, incuding Mishna Berura who goes LKULA like tosfos (see siman 9, s”k 14 very well vdoik) I happen to think that tos. is the pashtus of the sugya, and most likely the correct girsa in the sifri.
Rav Belskey and Rav Shachter both pasken to do like tosfos.
on tekehlet.net (a mirror of the old ptil website) in the “hot off the press” section there is a very good article on this, pro tosfos.
however, the majority of people who wear techeiles do raavad but Their reasons all vary. Some think that we look at psukim and if we think it makes sense that “psil” means only one, than that is the halacha (even if 30+ rishonim say there is two strings). Some people are convinced that the gra holds like the raavad (even though in biur hagra to shulchan aruch the gra paskens very strongly that there is 2 strings).
Basically it is a complicated subject, and much has been written in every direction. take a look at the library on techeiles.org for many articles.