The ultra-fine clay is meant for those who know they have very small and few surface contaminants, or people who like to clay more frequently (4 or more times per year). If you're just looking for clay to use a couple times annually, or on a vehicle which really needs to be clayed, go with the Block clay.
Unless the ultra-fine is what you NEED, I'd just go with the Clay Magic kit or Mother's clay - regardless of what my display name is
The Block clay isn't really worth the extra $$.