Apart from the prices, probably each manufacturer comes up with its own combination of clay & lube to stake its claim, whatever that may be. For instance, the Zaino clay kit receives much praises on another SUV site, but some gurus in Autopia may describe it as a bit too hard. The Meguiar's kit I used seems much softer and has pretty good "pick up" power; however, it does smear easily. Although the Mothers' clay that I tried seems a bit more "rigid" than the Mequiar's and requires more passes to clean the same surface, it doesn't turn gooey and needs less effort to clean up afterwards. I had positive experiences with both and would get whichever one is on sale locally. The much raved ClayMagic bars are not quite available in Hawaii. Best bet perhaps is to try the less expensive ones first. Unless it doesn't work for your lube+paint condition+technique combination; then, try the more pricey brands.
Aloha!
:wavey :dance