The first thing to point out is that both of these products are part of the Dermalogica UltraCalming range. The range and these products have been created to deal with sensitivity. It is designed to address the top three concerns of people with sensitive skin: redness, discomfort and dehydration. So basically both UltraCalming Mist and Redness Relief Essence will both reduce skin redness and calm sensitivity. They do however differ in their texture, how they work and how you apply them.
The second thing to point out is that UltraCalming Mist is a toner and Redness Relief Essence is an essence.