Assuming they're legit, they're all equally potent on a mg for mg basis. The concentration of mg per iu varies in different brands and that is usually why people feel one brand is "stronger". Also, I beleive the first three you mentioned are all sold in a liqud state, thus making it much more fragile and subject to more stringent temperature requirements. Serostim is freeze dried, so it is much more suitable for the underground market. Make sure your source's kits are the domestic holograms.