Not sure about the Belkin but;
Zum frost = 1mm
iBlason = 1.2 mm
Splitting hairs here really so doubt you'd even notice that small a difference. I use the Iblason myself and really dig it. Don't think you can go wrong with either choice. Considering all these kinds of cases are meant to be thin/lightweight, I sincerely doubt you'd find any great difference in thickness between them. Stats wise, the Zum frost is the thinnest.